diff --git a/Cargo.lock b/Cargo.lock
index c9eb6f34..b88398a3 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -497,7 +497,6 @@ version = "0.1.0"
 dependencies = [
  "fxhash",
  "lazy_static",
- "num-bigint 0.4.3",
  "parking_lot",
  "string-interner",
 ]
@@ -512,7 +511,6 @@ dependencies = [
  "insta",
  "itertools",
  "nac3parser",
- "num-bigint 0.3.3",
  "num-traits",
  "parking_lot",
  "rayon",
@@ -529,7 +527,6 @@ dependencies = [
  "lalrpop-util",
  "log",
  "nac3ast",
- "num-bigint 0.4.3",
  "num-traits",
  "phf",
  "unic-emoji-char",
@@ -553,38 +550,6 @@ version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e4a24736216ec316047a1fc4252e27dabb04218aa4a3f37c6e7ddbf1f9782b54"
 
-[[package]]
-name = "num-bigint"
-version = "0.3.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5f6f7833f2cbf2360a6cfd58cd41a53aa7a90bd4c202f5b1c7dd2ed73c57b2c3"
-dependencies = [
- "autocfg",
- "num-integer",
- "num-traits",
-]
-
-[[package]]
-name = "num-bigint"
-version = "0.4.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f93ab6289c7b344a8a9f60f88d80aa20032336fe78da341afc91c8a2341fc75f"
-dependencies = [
- "autocfg",
- "num-integer",
- "num-traits",
-]
-
-[[package]]
-name = "num-integer"
-version = "0.1.44"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d2cc698a63b549a70bc047073d2949cce27cd1c7b0a4a862d08a8031bc2801db"
-dependencies = [
- "autocfg",
- "num-traits",
-]
-
 [[package]]
 name = "num-traits"
 version = "0.2.14"
diff --git a/nac3ast/Cargo.toml b/nac3ast/Cargo.toml
index 5e0da134..802a0bae 100644
--- a/nac3ast/Cargo.toml
+++ b/nac3ast/Cargo.toml
@@ -10,7 +10,6 @@ constant-optimization = ["fold"]
 fold = []
 
 [dependencies]
-num-bigint = "0.4.0"
 lazy_static = "1.4.0"
 parking_lot = "0.11.1"
 string-interner = "0.13.0"
diff --git a/nac3ast/src/constant.rs b/nac3ast/src/constant.rs
index 3e5ed8d7..b3a7ed58 100644
--- a/nac3ast/src/constant.rs
+++ b/nac3ast/src/constant.rs
@@ -1,12 +1,10 @@
-use num_bigint::BigInt;
-
 #[derive(Clone, Debug, PartialEq)]
 pub enum Constant {
     None,
     Bool(bool),
     Str(String),
     Bytes(Vec<u8>),
-    Int(BigInt),
+    Int(Option<i64>),
     Tuple(Vec<Constant>),
     Float(f64),
     Complex { real: f64, imag: f64 },
@@ -28,9 +26,14 @@ impl From<bool> for Constant {
         Self::Bool(b)
     }
 }
-impl From<BigInt> for Constant {
-    fn from(i: BigInt) -> Constant {
-        Self::Int(i)
+impl From<i32> for Constant {
+    fn from(i: i32) -> Constant {
+        Self::Int(Some(i as i64))
+    }
+}
+impl From<i64> for Constant {
+    fn from(i: i64) -> Constant {
+        Self::Int(Some(i))
     }
 }
 
@@ -136,7 +139,7 @@ mod tests {
                         location,
                         custom,
                         node: ExprKind::Constant {
-                            value: BigInt::from(1).into(),
+                            value: 1.into(),
                             kind: None,
                         },
                     },
@@ -144,7 +147,7 @@ mod tests {
                         location,
                         custom,
                         node: ExprKind::Constant {
-                            value: BigInt::from(2).into(),
+                            value: 2.into(),
                             kind: None,
                         },
                     },
@@ -158,7 +161,7 @@ mod tests {
                                     location,
                                     custom,
                                     node: ExprKind::Constant {
-                                        value: BigInt::from(3).into(),
+                                        value: 3.into(),
                                         kind: None,
                                     },
                                 },
@@ -166,7 +169,7 @@ mod tests {
                                     location,
                                     custom,
                                     node: ExprKind::Constant {
-                                        value: BigInt::from(4).into(),
+                                        value: 4.into(),
                                         kind: None,
                                     },
                                 },
@@ -174,7 +177,7 @@ mod tests {
                                     location,
                                     custom,
                                     node: ExprKind::Constant {
-                                        value: BigInt::from(5).into(),
+                                        value: 5.into(),
                                         kind: None,
                                     },
                                 },
@@ -194,12 +197,12 @@ mod tests {
                 custom,
                 node: ExprKind::Constant {
                     value: Constant::Tuple(vec![
-                        BigInt::from(1).into(),
-                        BigInt::from(2).into(),
+                        1.into(),
+                        2.into(),
                         Constant::Tuple(vec![
-                            BigInt::from(3).into(),
-                            BigInt::from(4).into(),
-                            BigInt::from(5).into(),
+                            3.into(),
+                            4.into(),
+                            5.into(),
                         ])
                     ]),
                     kind: None
diff --git a/nac3core/Cargo.toml b/nac3core/Cargo.toml
index 86b23ea0..2c508979 100644
--- a/nac3core/Cargo.toml
+++ b/nac3core/Cargo.toml
@@ -5,7 +5,6 @@ authors = ["M-Labs"]
 edition = "2018"
 
 [dependencies]
-num-bigint = "0.3"
 num-traits = "0.2"
 itertools = "0.10.1"
 crossbeam = "0.8.1"
diff --git a/nac3core/src/codegen/expr.rs b/nac3core/src/codegen/expr.rs
index 940c9df7..69c2423a 100644
--- a/nac3core/src/codegen/expr.rs
+++ b/nac3core/src/codegen/expr.rs
@@ -123,7 +123,7 @@ impl<'ctx, 'a> CodeGenContext<'ctx, 'a> {
                 let ty = self.ctx.bool_type();
                 ty.const_int(if *v { 1 } else { 0 }, false).into()
             }
-            Constant::Int(v) => {
+            Constant::Int(Some(val)) => {
                 let ty = if self.unifier.unioned(ty, self.primitives.int32) {
                     self.ctx.i32_type()
                 } else if self.unifier.unioned(ty, self.primitives.int64) {
@@ -131,8 +131,7 @@ impl<'ctx, 'a> CodeGenContext<'ctx, 'a> {
                 } else {
                     unreachable!();
                 };
-                let val: i64 = v.try_into().unwrap();
-                ty.const_int(val as u64, false).into()
+                ty.const_int(*val as u64, false).into()
             }
             Constant::Float(v) => {
                 assert!(self.unifier.unioned(ty, self.primitives.float));
diff --git a/nac3core/src/toplevel/helper.rs b/nac3core/src/toplevel/helper.rs
index b9259854..dcc74dd1 100644
--- a/nac3core/src/toplevel/helper.rs
+++ b/nac3core/src/toplevel/helper.rs
@@ -412,13 +412,23 @@ pub fn parse_parameter_default_value(default: &ast::Expr, resolver: &(dyn Symbol
     fn handle_constant(val: &Constant, loc: &Location) -> Result<SymbolValue, String> {
         match val {
             Constant::Int(v) => {
-                if let Ok(v) = v.try_into() {
-                    Ok(SymbolValue::I32(v))
-                } else {
-                    Err(format!(
-                        "integer value out of range at {}",
-                        loc
-                    ))
+                match v {
+                    Some(v) => {
+                        if let Ok(v) = (*v).try_into() {
+                            Ok(SymbolValue::I32(v))
+                        } else {
+                            Err(format!(
+                                "integer value out of range at {}",
+                                loc
+                            ))
+                        }
+                    },
+                    None => {
+                        Err(format!(
+                            "integer value out of range at {}",
+                            loc
+                        ))
+                    }
                 }
             }
             Constant::Float(v) => Ok(SymbolValue::Double(*v)),
@@ -439,8 +449,8 @@ pub fn parse_parameter_default_value(default: &ast::Expr, resolver: &(dyn Symbol
         } => {
             if args.len() == 1 {
                 match &args[0].node {
-                    ast::ExprKind::Constant { value: Constant::Int(v), .. } =>
-                        Ok(SymbolValue::I64(v.try_into().unwrap())),
+                    ast::ExprKind::Constant { value: Constant::Int(Some(v)), .. } =>
+                        Ok(SymbolValue::I64(*v)),
                     _ => Err(format!("only allow constant integer here at {}", default.location))
                 }
             } else {
diff --git a/nac3core/src/typecheck/type_inferencer/mod.rs b/nac3core/src/typecheck/type_inferencer/mod.rs
index c18bed89..ad32baaa 100644
--- a/nac3core/src/typecheck/type_inferencer/mod.rs
+++ b/nac3core/src/typecheck/type_inferencer/mod.rs
@@ -680,11 +680,8 @@ impl<'a> Inferencer<'a> {
                     if let ExprKind::Constant { value: ast::Constant::Int(val), kind } =
                         &args[0].node
                     {
-                        let int64: Result<i64, _> = val.try_into();
-                        let custom;
-                        if int64.is_ok() {
-                            custom = Some(self.primitives.int64);
-                        } else {
+                        let custom = Some(self.primitives.int64);
+                        if val.is_none() {
                             return Err("Integer out of bound".into());
                         }
                         return Ok(Located {
@@ -777,12 +774,17 @@ impl<'a> Inferencer<'a> {
         match constant {
             ast::Constant::Bool(_) => Ok(self.primitives.bool),
             ast::Constant::Int(val) => {
-                let int32: Result<i32, _> = val.try_into();
-                // int64 would be handled separately in functions
-                if int32.is_ok() {
-                    Ok(self.primitives.int32)
-                } else {
-                    Err("Integer out of bound".into())
+                match val {
+                    Some(val) => {
+                        let int32: Result<i32, _> = (*val).try_into();
+                        // int64 is handled separately in functions
+                        if int32.is_ok() {
+                            Ok(self.primitives.int32)
+                        } else {
+                            Err("Integer out of bound".into())
+                        }
+                    },
+                    None => Err("Integer out of bound".into())
                 }
             }
             ast::Constant::Float(_) => Ok(self.primitives.float),
@@ -907,7 +909,11 @@ impl<'a> Inferencer<'a> {
             }
             ast::ExprKind::Constant { value: ast::Constant::Int(val), .. } => {
                 // the index is a constant, so value can be a sequence.
-                let ind: i32 = val.try_into().map_err(|_| "Index must be int32".to_string())?;
+                let ind: Option<i32> = match val {
+                    Some(val) => (*val).try_into().ok(),
+                    None => None,
+                };
+                let ind = ind.ok_or_else(|| "Index must be int32".to_string())?;
                 let map = once((ind, ty)).collect();
                 let seq = self.unifier.add_sequence(map);
                 self.constrain(value.custom.unwrap(), seq, &value.location)?;
diff --git a/nac3parser/Cargo.toml b/nac3parser/Cargo.toml
index 1db1b1bb..34e94284 100644
--- a/nac3parser/Cargo.toml
+++ b/nac3parser/Cargo.toml
@@ -14,7 +14,6 @@ lalrpop = "0.19.6"
 nac3ast = { path = "../nac3ast" }
 lalrpop-util = "0.19.6"
 log = "0.4.1"
-num-bigint = "0.4.0"
 num-traits = "0.2"
 unic-emoji-char = "0.9"
 unic-ucd-ident  = "0.9"
diff --git a/nac3parser/src/lexer.rs b/nac3parser/src/lexer.rs
index 01dfabaa..07227955 100644
--- a/nac3parser/src/lexer.rs
+++ b/nac3parser/src/lexer.rs
@@ -5,12 +5,11 @@
 pub use super::token::Tok;
 use crate::ast::{Location, FileName};
 use crate::error::{LexicalError, LexicalErrorType};
-use num_bigint::BigInt;
 use num_traits::identities::Zero;
-use num_traits::Num;
 use std::char;
 use std::cmp::Ordering;
 use std::str::FromStr;
+use std::num::IntErrorKind;
 use unic_emoji_char::is_emoji_presentation;
 use unic_ucd_ident::{is_xid_continue, is_xid_start};
 
@@ -287,10 +286,18 @@ where
     fn lex_number_radix(&mut self, start_pos: Location, radix: u32) -> LexResult {
         let value_text = self.radix_run(radix);
         let end_pos = self.get_pos();
-        let value = BigInt::from_str_radix(&value_text, radix).map_err(|e| LexicalError {
-            error: LexicalErrorType::OtherError(format!("{:?}", e)),
-            location: start_pos,
-        })?;
+        let value = match i64::from_str_radix(&value_text, radix) {
+            Ok(value) => Some(value),
+            Err(e) => {
+                match e.kind() {
+                    IntErrorKind::PosOverflow | IntErrorKind::NegOverflow => None,
+                    _ => return Err(LexicalError {
+                        error: LexicalErrorType::OtherError(format!("{:?}", e)),
+                        location: start_pos,
+                    }),
+                }
+            }
+        };
         Ok((start_pos, Tok::Int { value }, end_pos))
     }
 
@@ -353,8 +360,14 @@ where
                 Ok((start_pos, Tok::Complex { real: 0.0, imag }, end_pos))
             } else {
                 let end_pos = self.get_pos();
-                let value = value_text.parse::<BigInt>().unwrap();
-                if start_is_zero && !value.is_zero() {
+                // assumption: value_text contains a valid integer.
+                // parse should only fail because of overflow.
+                let value = value_text.parse::<i64>().ok();
+                let nonzero = match value {
+                    Some(value) => !value.is_zero(),
+                    None => true
+                };
+                if start_is_zero && nonzero {
                     return Err(LexicalError {
                         error: LexicalErrorType::OtherError("Invalid Token".to_owned()),
                         location: self.get_pos(),
@@ -1323,7 +1336,6 @@ mod tests {
     use std::default;
 
     use super::{make_tokenizer, NewlineHandler, Tok};
-    use num_bigint::BigInt;
 
     const WINDOWS_EOL: &str = "\r\n";
     const MAC_EOL: &str = "\r";
@@ -1451,16 +1463,16 @@ class Foo(A, B):
             tokens,
             vec![
                 Tok::Int {
-                    value: BigInt::from(47),
+                    value: Some(47i64),
                 },
                 Tok::Int {
-                    value: BigInt::from(13),
+                    value: Some(13i64),
                 },
                 Tok::Int {
-                    value: BigInt::from(0),
+                    value: Some(0i64),
                 },
                 Tok::Int {
-                    value: BigInt::from(123),
+                    value: Some(123i64),
                 },
                 Tok::Float { value: 0.2 },
                 Tok::Complex {
@@ -1483,7 +1495,7 @@ class Foo(A, B):
             fn $name() {
                 let source = format!(r"99232  # {}", $eol);
                 let tokens = lex_source(&source);
-                assert_eq!(tokens, vec![Tok::Int { value: BigInt::from(99232) }, Tok::Newline]);
+                assert_eq!(tokens, vec![Tok::Int { value: Some(99232i64) }, Tok::Newline]);
             }
             )*
         }
@@ -1506,9 +1518,9 @@ class Foo(A, B):
                 assert_eq!(
                     tokens,
                     vec![
-                        Tok::Int { value: BigInt::from(123) },
+                        Tok::Int { value: Some(123i64) },
                         Tok::Newline,
-                        Tok::Int { value: BigInt::from(456) },
+                        Tok::Int { value: Some(456i64) },
                         Tok::Newline,
                     ]
                 )
@@ -1535,15 +1547,15 @@ class Foo(A, B):
                 },
                 Tok::Equal,
                 Tok::Int {
-                    value: BigInt::from(99)
+                    value: Some(99i64)
                 },
                 Tok::Plus,
                 Tok::Int {
-                    value: BigInt::from(2)
+                    value: Some(2i64)
                 },
                 Tok::Minus,
                 Tok::Int {
-                    value: BigInt::from(0)
+                    value: Some(0i64)
                 },
                 Tok::Newline,
             ]
@@ -1570,7 +1582,7 @@ class Foo(A, B):
                         Tok::Newline,
                         Tok::Indent,
                         Tok::Return,
-                        Tok::Int { value: BigInt::from(99) },
+                        Tok::Int { value: Some(99i64) },
                         Tok::Newline,
                         Tok::Dedent,
                     ]
@@ -1613,7 +1625,7 @@ class Foo(A, B):
                         Tok::Newline,
                         Tok::Indent,
                         Tok::Return,
-                        Tok::Int { value: BigInt::from(99) },
+                        Tok::Int { value: Some(99i64) },
                         Tok::Newline,
                         Tok::Dedent,
                         Tok::Dedent,
@@ -1651,7 +1663,7 @@ class Foo(A, B):
                         Tok::Newline,
                         Tok::Indent,
                         Tok::Return,
-                        Tok::Int { value: BigInt::from(99) },
+                        Tok::Int { value: Some(99i64) },
                         Tok::Newline,
                         Tok::Dedent,
                         Tok::Dedent,
@@ -1689,9 +1701,9 @@ class Foo(A, B):
                         },
                         Tok::Equal,
                         Tok::Lsqb,
-                        Tok::Int { value: BigInt::from(1) },
+                        Tok::Int { value: Some(1i64) },
                         Tok::Comma,
-                        Tok::Int { value: BigInt::from(2) },
+                        Tok::Int { value: Some(2i64) },
                         Tok::Rsqb,
                         Tok::Newline,
                     ]
diff --git a/nac3parser/src/parser.rs b/nac3parser/src/parser.rs
index 1d25858d..29a8fbf1 100644
--- a/nac3parser/src/parser.rs
+++ b/nac3parser/src/parser.rs
@@ -31,7 +31,6 @@ pub fn parse_program(source: &str, file: FileName) -> Result<ast::Suite, ParseEr
 ///
 /// # Example
 /// ```
-/// extern crate num_bigint;
 /// use nac3parser::{parser, ast};
 /// let expr = parser::parse_expression("1 + 2").unwrap();
 ///
diff --git a/nac3parser/src/python.lalrpop b/nac3parser/src/python.lalrpop
index 7cfdf3a3..41420ab5 100644
--- a/nac3parser/src/python.lalrpop
+++ b/nac3parser/src/python.lalrpop
@@ -14,7 +14,6 @@ use crate::lexer;
 use crate::config_comment_helper::*;
 
 use lalrpop_util::ParseError;
-use num_bigint::BigInt;
 
 grammar;
 
@@ -920,7 +919,7 @@ Factor: ast::Expr = {
             match (&op, &e.node) {
                 (ast::Unaryop::USub, ast::ExprKind::Constant { value: Constant::Int(val), kind }) => {
                     ast::ExprKind::Constant {
-                        value: Constant::Int(-val),
+                        value: if let Some(val) = val { Constant::Int(Some(-val)) } else { Constant::Int(None) },
                         kind: kind.clone()
                     }
                 }
@@ -1362,7 +1361,7 @@ extern {
         "True" => lexer::Tok::True,
         "False" => lexer::Tok::False,
         "None" => lexer::Tok::None,
-        int => lexer::Tok::Int { value: <BigInt> },
+        int => lexer::Tok::Int { value: <Option<i64>> },
         float => lexer::Tok::Float { value: <f64> },
         complex => lexer::Tok::Complex { real: <f64>, imag: <f64> },
         string => lexer::Tok::String { value: <String>, is_fstring: <bool> },
diff --git a/nac3parser/src/snapshots/nac3parser__fstring__tests__parse_fstring_equals.snap b/nac3parser/src/snapshots/nac3parser__fstring__tests__parse_fstring_equals.snap
index c1215ed6..6514281b 100644
--- a/nac3parser/src/snapshots/nac3parser__fstring__tests__parse_fstring_equals.snap
+++ b/nac3parser/src/snapshots/nac3parser__fstring__tests__parse_fstring_equals.snap
@@ -1,5 +1,6 @@
 ---
-source: parser/src/fstring.rs
+source: nac3parser/src/fstring.rs
+assertion_line: 382
 expression: parse_ast
 
 ---
@@ -25,7 +26,9 @@ Located {
                     custom: (),
                     node: Constant {
                         value: Int(
-                            42,
+                            Some(
+                                42,
+                            ),
                         ),
                         kind: None,
                     },
@@ -42,7 +45,9 @@ Located {
                         custom: (),
                         node: Constant {
                             value: Int(
-                                42,
+                                Some(
+                                    42,
+                                ),
                             ),
                             kind: None,
                         },
diff --git a/nac3parser/src/snapshots/nac3parser__fstring__tests__parse_fstring_not_equals.snap b/nac3parser/src/snapshots/nac3parser__fstring__tests__parse_fstring_not_equals.snap
index 74bf50b5..2eb3b3e6 100644
--- a/nac3parser/src/snapshots/nac3parser__fstring__tests__parse_fstring_not_equals.snap
+++ b/nac3parser/src/snapshots/nac3parser__fstring__tests__parse_fstring_not_equals.snap
@@ -1,5 +1,6 @@
 ---
-source: parser/src/fstring.rs
+source: nac3parser/src/fstring.rs
+assertion_line: 375
 expression: parse_ast
 
 ---
@@ -25,7 +26,9 @@ Located {
                     custom: (),
                     node: Constant {
                         value: Int(
-                            1,
+                            Some(
+                                1,
+                            ),
                         ),
                         kind: None,
                     },
@@ -42,7 +45,9 @@ Located {
                         custom: (),
                         node: Constant {
                             value: Int(
-                                2,
+                                Some(
+                                    2,
+                                ),
                             ),
                             kind: None,
                         },
diff --git a/nac3parser/src/snapshots/nac3parser__parser__tests__comment_ambiguity.snap b/nac3parser/src/snapshots/nac3parser__parser__tests__comment_ambiguity.snap
index e46deac7..3886734a 100644
--- a/nac3parser/src/snapshots/nac3parser__parser__tests__comment_ambiguity.snap
+++ b/nac3parser/src/snapshots/nac3parser__parser__tests__comment_ambiguity.snap
@@ -1,5 +1,6 @@
 ---
 source: nac3parser/src/parser.rs
+assertion_line: 218
 expression: parse_program(&source).unwrap()
 
 ---
@@ -172,7 +173,9 @@ expression: parse_program(&source).unwrap()
                                     custom: (),
                                     node: Constant {
                                         value: Int(
-                                            3,
+                                            Some(
+                                                3,
+                                            ),
                                         ),
                                         kind: None,
                                     },
@@ -219,7 +222,9 @@ expression: parse_program(&source).unwrap()
                 custom: (),
                 node: Constant {
                     value: Int(
-                        3,
+                        Some(
+                            3,
+                        ),
                     ),
                     kind: None,
                 },
@@ -262,7 +267,9 @@ expression: parse_program(&source).unwrap()
                         custom: (),
                         node: Constant {
                             value: Int(
-                                3,
+                                Some(
+                                    3,
+                                ),
                             ),
                             kind: None,
                         },
@@ -337,7 +344,9 @@ expression: parse_program(&source).unwrap()
                 custom: (),
                 node: Constant {
                     value: Int(
-                        3,
+                        Some(
+                            3,
+                        ),
                     ),
                     kind: None,
                 },
diff --git a/nac3parser/src/snapshots/nac3parser__parser__tests__more_comment.snap b/nac3parser/src/snapshots/nac3parser__parser__tests__more_comment.snap
index 790bb7f3..918c41f1 100644
--- a/nac3parser/src/snapshots/nac3parser__parser__tests__more_comment.snap
+++ b/nac3parser/src/snapshots/nac3parser__parser__tests__more_comment.snap
@@ -1,5 +1,6 @@
 ---
 source: nac3parser/src/parser.rs
+assertion_line: 186
 expression: parse_program(&source).unwrap()
 
 ---
@@ -74,7 +75,9 @@ expression: parse_program(&source).unwrap()
                             custom: (),
                             node: Constant {
                                 value: Int(
-                                    1,
+                                    Some(
+                                        1,
+                                    ),
                                 ),
                                 kind: None,
                             },
@@ -177,7 +180,9 @@ expression: parse_program(&source).unwrap()
                             custom: (),
                             node: Constant {
                                 value: Int(
-                                    2,
+                                    Some(
+                                        2,
+                                    ),
                                 ),
                                 kind: None,
                             },
@@ -220,7 +225,9 @@ expression: parse_program(&source).unwrap()
                                     custom: (),
                                     node: Constant {
                                         value: Int(
-                                            1,
+                                            Some(
+                                                1,
+                                            ),
                                         ),
                                         kind: None,
                                     },
@@ -234,7 +241,9 @@ expression: parse_program(&source).unwrap()
                                     custom: (),
                                     node: Constant {
                                         value: Int(
-                                            2,
+                                            Some(
+                                                2,
+                                            ),
                                         ),
                                         kind: None,
                                     },
@@ -263,7 +272,9 @@ expression: parse_program(&source).unwrap()
                             custom: (),
                             node: Constant {
                                 value: Int(
-                                    1,
+                                    Some(
+                                        1,
+                                    ),
                                 ),
                                 kind: None,
                             },
@@ -284,7 +295,9 @@ expression: parse_program(&source).unwrap()
                                         custom: (),
                                         node: Constant {
                                             value: Int(
-                                                3,
+                                                Some(
+                                                    3,
+                                                ),
                                             ),
                                             kind: None,
                                         },
diff --git a/nac3parser/src/snapshots/nac3parser__parser__tests__parse_double_list_comprehension.snap b/nac3parser/src/snapshots/nac3parser__parser__tests__parse_double_list_comprehension.snap
index 4095f648..59b29a1f 100644
--- a/nac3parser/src/snapshots/nac3parser__parser__tests__parse_double_list_comprehension.snap
+++ b/nac3parser/src/snapshots/nac3parser__parser__tests__parse_double_list_comprehension.snap
@@ -1,5 +1,6 @@
 ---
-source: parser/src/parser.rs
+source: nac3parser/src/parser.rs
+assertion_line: 164
 expression: parse_ast
 
 ---
@@ -125,7 +126,9 @@ Located {
                                     custom: (),
                                     node: Constant {
                                         value: Int(
-                                            5,
+                                            Some(
+                                                5,
+                                            ),
                                         ),
                                         kind: None,
                                     },
@@ -163,7 +166,9 @@ Located {
                                     custom: (),
                                     node: Constant {
                                         value: Int(
-                                            10,
+                                            Some(
+                                                10,
+                                            ),
                                         ),
                                         kind: None,
                                     },
diff --git a/nac3parser/src/snapshots/nac3parser__parser__tests__parse_if_elif_else.snap b/nac3parser/src/snapshots/nac3parser__parser__tests__parse_if_elif_else.snap
index 13f0bca8..a7a9b072 100644
--- a/nac3parser/src/snapshots/nac3parser__parser__tests__parse_if_elif_else.snap
+++ b/nac3parser/src/snapshots/nac3parser__parser__tests__parse_if_elif_else.snap
@@ -1,5 +1,6 @@
 ---
 source: nac3parser/src/parser.rs
+assertion_line: 118
 expression: parse_ast
 
 ---
@@ -19,7 +20,9 @@ expression: parse_ast
                 custom: (),
                 node: Constant {
                     value: Int(
-                        1,
+                        Some(
+                            1,
+                        ),
                     ),
                     kind: None,
                 },
@@ -40,7 +43,9 @@ expression: parse_ast
                             custom: (),
                             node: Constant {
                                 value: Int(
-                                    10,
+                                    Some(
+                                        10,
+                                    ),
                                 ),
                                 kind: None,
                             },
@@ -65,7 +70,9 @@ expression: parse_ast
                             custom: (),
                             node: Constant {
                                 value: Int(
-                                    2,
+                                    Some(
+                                        2,
+                                    ),
                                 ),
                                 kind: None,
                             },
@@ -86,7 +93,9 @@ expression: parse_ast
                                         custom: (),
                                         node: Constant {
                                             value: Int(
-                                                20,
+                                                Some(
+                                                    20,
+                                                ),
                                             ),
                                             kind: None,
                                         },
@@ -111,7 +120,9 @@ expression: parse_ast
                                         custom: (),
                                         node: Constant {
                                             value: Int(
-                                                30,
+                                                Some(
+                                                    30,
+                                                ),
                                             ),
                                             kind: None,
                                         },
diff --git a/nac3parser/src/snapshots/nac3parser__parser__tests__parse_kwargs.snap b/nac3parser/src/snapshots/nac3parser__parser__tests__parse_kwargs.snap
index 6dd94aae..cde4b126 100644
--- a/nac3parser/src/snapshots/nac3parser__parser__tests__parse_kwargs.snap
+++ b/nac3parser/src/snapshots/nac3parser__parser__tests__parse_kwargs.snap
@@ -1,5 +1,6 @@
 ---
 source: nac3parser/src/parser.rs
+assertion_line: 111
 expression: parse_ast
 
 ---
@@ -63,7 +64,9 @@ expression: parse_ast
                                     custom: (),
                                     node: Constant {
                                         value: Int(
-                                            2,
+                                            Some(
+                                                2,
+                                            ),
                                         ),
                                         kind: None,
                                     },
diff --git a/nac3parser/src/snapshots/nac3parser__parser__tests__parse_print_2.snap b/nac3parser/src/snapshots/nac3parser__parser__tests__parse_print_2.snap
index 92e1c949..83f5ab02 100644
--- a/nac3parser/src/snapshots/nac3parser__parser__tests__parse_print_2.snap
+++ b/nac3parser/src/snapshots/nac3parser__parser__tests__parse_print_2.snap
@@ -1,5 +1,6 @@
 ---
 source: nac3parser/src/parser.rs
+assertion_line: 104
 expression: parse_ast
 
 ---
@@ -51,7 +52,9 @@ expression: parse_ast
                             custom: (),
                             node: Constant {
                                 value: Int(
-                                    2,
+                                    Some(
+                                        2,
+                                    ),
                                 ),
                                 kind: None,
                             },
diff --git a/nac3parser/src/snapshots/nac3parser__parser__tests__parse_tuples.snap b/nac3parser/src/snapshots/nac3parser__parser__tests__parse_tuples.snap
index 66f2fdd6..90ad38f6 100644
--- a/nac3parser/src/snapshots/nac3parser__parser__tests__parse_tuples.snap
+++ b/nac3parser/src/snapshots/nac3parser__parser__tests__parse_tuples.snap
@@ -1,5 +1,6 @@
 ---
 source: nac3parser/src/parser.rs
+assertion_line: 132
 expression: parse_program(&source).unwrap()
 
 ---
@@ -63,7 +64,9 @@ expression: parse_program(&source).unwrap()
                             custom: (),
                             node: Constant {
                                 value: Int(
-                                    4,
+                                    Some(
+                                        4,
+                                    ),
                                 ),
                                 kind: None,
                             },
@@ -76,7 +79,9 @@ expression: parse_program(&source).unwrap()
                             custom: (),
                             node: Constant {
                                 value: Int(
-                                    5,
+                                    Some(
+                                        5,
+                                    ),
                                 ),
                                 kind: None,
                             },
diff --git a/nac3parser/src/snapshots/nac3parser__parser__tests__sample_comment.snap b/nac3parser/src/snapshots/nac3parser__parser__tests__sample_comment.snap
index 8e34a64f..69ce7975 100644
--- a/nac3parser/src/snapshots/nac3parser__parser__tests__sample_comment.snap
+++ b/nac3parser/src/snapshots/nac3parser__parser__tests__sample_comment.snap
@@ -1,5 +1,6 @@
 ---
 source: nac3parser/src/parser.rs
+assertion_line: 199
 expression: parse_program(&source).unwrap()
 
 ---
@@ -51,7 +52,9 @@ expression: parse_program(&source).unwrap()
                             custom: (),
                             node: Constant {
                                 value: Int(
-                                    3,
+                                    Some(
+                                        3,
+                                    ),
                                 ),
                                 kind: None,
                             },
diff --git a/nac3parser/src/token.rs b/nac3parser/src/token.rs
index 163412aa..c82ad153 100644
--- a/nac3parser/src/token.rs
+++ b/nac3parser/src/token.rs
@@ -1,6 +1,5 @@
 //! Different token definitions.
 //! Loosely based on token.h from CPython source:
-use num_bigint::BigInt;
 use std::fmt::{self, Write};
 use crate::ast;
 
@@ -8,7 +7,7 @@ use crate::ast;
 #[derive(Clone, Debug, PartialEq)]
 pub enum Tok {
     Name { name: ast::StrRef },
-    Int { value: BigInt },
+    Int { value: Option<i64> },
     Float { value: f64 },
     Complex { real: f64, imag: f64 },
     String { value: String, is_fstring: bool },
@@ -113,7 +112,7 @@ impl fmt::Display for Tok {
         use Tok::*;
         match self {
             Name { name } => write!(f, "'{}'", ast::get_str_from_ref(&ast::get_str_ref_lock(), *name)),
-            Int { value } => write!(f, "'{}'", value),
+            Int { value } => if let Some(value) = value { write!(f, "'{}'", value) } else { write!(f, "'#OFL#'") },
             Float { value } => write!(f, "'{}'", value),
             Complex { real, imag } => write!(f, "{}j{}", real, imag),
             String { value, is_fstring } => {