diff --git a/nac3core/src/typecheck/type_inferencer/mod.rs b/nac3core/src/typecheck/type_inferencer/mod.rs
index a6b72bb..f1f2a5a 100644
--- a/nac3core/src/typecheck/type_inferencer/mod.rs
+++ b/nac3core/src/typecheck/type_inferencer/mod.rs
@@ -13,7 +13,7 @@ use crate::{
         TopLevelContext,
     },
 };
-use itertools::izip;
+use itertools::{Itertools, izip};
 use nac3parser::ast::{
     self,
     fold::{self, Fold},
@@ -59,6 +59,16 @@ pub struct PrimitiveStore {
 }
 
 impl PrimitiveStore {
+    /// Returns a [`Type`] representing a signed representation of `size_t`.
+    #[must_use]
+    pub fn isize(&self) -> Type {
+        match self.size_t {
+            32 => self.int32,
+            64 => self.int64,
+            _ => unreachable!(),
+        }
+    }
+
     /// Returns a [Type] representing `size_t`.
     #[must_use]
     pub fn usize(&self) -> Type {
@@ -1381,7 +1391,16 @@ impl<'a> Inferencer<'a> {
                     TypeEnum::TObj { obj_id, .. } if *obj_id == PRIMITIVE_DEF_IDS.ndarray => {
                         let (_, ndims) = unpack_ndarray_tvars(self.unifier, value.custom.unwrap());
 
-                        self.constrain(slice.custom.unwrap(), self.primitives.usize(), &slice.location)?;
+                        let valid_index_tys = [
+                            self.primitives.int32,
+                            self.primitives.isize(),
+                        ].into_iter().unique().collect_vec();
+                        let valid_index_ty = self.unifier.get_fresh_var_with_range(
+                            valid_index_tys.as_slice(),
+                            None,
+                            None,
+                        ).0;
+                        self.constrain(slice.custom.unwrap(), valid_index_ty, &slice.location)?;
                         self.infer_subscript_ndarray(value, ty, ndims)
                     }
                     _ => unreachable!(),