diff --git a/nac3core/build.rs b/nac3core/build.rs
index 345b042b..2f92281e 100644
--- a/nac3core/build.rs
+++ b/nac3core/build.rs
@@ -24,6 +24,12 @@ fn compile_irrt_cpp() {
     let out_dir = get_out_dir();
     let irrt_dir = get_irrt_dir();
 
+    let (opt_flag, debug_assert_flag) = match env::var("PROFILE").as_deref() {
+        Ok("debug") => ("-O0", "-DIRRT_DEBUG_ASSERT=true"),
+        Ok("release") => ("-O3", "-DIRRT_DEBUG_ASSERT=false"),
+        flavor => panic!("Unknown or missing build flavor {flavor:?}"),
+    };
+
     /*
      * HACK: Sadly, clang doesn't let us emit generic LLVM bitcode.
      * Compiling for WASM32 and filtering the output with regex is the closest we can get.
@@ -36,11 +42,8 @@ fn compile_irrt_cpp() {
         "-fno-discard-value-names",
         "-fno-exceptions",
         "-fno-rtti",
-        match env::var("PROFILE").as_deref() {
-            Ok("debug") => "-O0",
-            Ok("release") => "-O3",
-            flavor => panic!("Unknown or missing build flavor {flavor:?}"),
-        },
+        opt_flag,
+        debug_assert_flag,
         "-emit-llvm",
         "-S",
         "-Wall",
diff --git a/nac3core/irrt/irrt/ndarray/indexing.hpp b/nac3core/irrt/irrt/ndarray/indexing.hpp
index c7328242..762866fe 100644
--- a/nac3core/irrt/irrt/ndarray/indexing.hpp
+++ b/nac3core/irrt/irrt/ndarray/indexing.hpp
@@ -71,13 +71,6 @@ namespace util {
 template <typename SizeT>
 SizeT validate_and_deduce_ndims_after_indexing(SizeT ndims, SizeT num_indexes,
                                                const NDIndex* indexes) {
-    if (num_indexes > ndims) {
-        raise_exception(SizeT, EXN_INDEX_ERROR,
-                        "too many indices for array: array is {0}-dimensional, "
-                        "but {1} were indexed",
-                        ndims, num_indexes, NO_PARAM);
-    }
-
     // There may be ellipsis `...` in `indexes`. There can only be 0 or 1 ellipsis.
     SizeT num_ellipsis = 0;
 
@@ -101,6 +94,14 @@ SizeT validate_and_deduce_ndims_after_indexing(SizeT ndims, SizeT num_indexes,
             __builtin_unreachable();
         }
     }
+
+    if (num_indexes - num_ellipsis > ndims) {
+        raise_exception(SizeT, EXN_INDEX_ERROR,
+                        "too many indices for array: array is {0}-dimensional, "
+                        "but {1} were indexed",
+                        ndims, num_indexes, NO_PARAM);
+    }
+
     return ndims;
 }
 }  // namespace util
@@ -139,9 +140,51 @@ SizeT validate_and_deduce_ndims_after_indexing(SizeT ndims, SizeT num_indexes,
 template <typename SizeT>
 void index(SizeT num_indexes, const NDIndex* indexes,
            const NDArray<SizeT>* src_ndarray, NDArray<SizeT>* dst_ndarray) {
-    SizeT expected_dst_ndarray_ndims =
-        util::validate_and_deduce_ndims_after_indexing(src_ndarray->ndims,
-                                                       num_indexes, indexes);
+    // First, validate `indexes`.
+
+    // Expected value of `dst_ndarray->ndims`.
+    SizeT expected_dst_ndims = src_ndarray->ndims;
+    // To check for "too many indices for array: array is ?-dimensional, but ? were indexed"
+    SizeT num_indexed = 0;
+    // There may be ellipsis `...` in `indexes`. There can only be 0 or 1 ellipsis.
+    SizeT num_ellipsis = 0;
+
+    for (SizeT i = 0; i < num_indexes; i++) {
+        if (indexes[i].type == ND_INDEX_TYPE_SINGLE_ELEMENT) {
+            expected_dst_ndims--;
+            num_indexed++;
+        } else if (indexes[i].type == ND_INDEX_TYPE_SLICE) {
+            num_indexed++;
+        } else if (indexes[i].type == ND_INDEX_TYPE_NEWAXIS) {
+            expected_dst_ndims++;
+        } else if (indexes[i].type == ND_INDEX_TYPE_ELLIPSIS) {
+            num_ellipsis++;
+            if (num_ellipsis > 1) {
+                raise_exception(
+                    SizeT, EXN_INDEX_ERROR,
+                    "an index can only have a single ellipsis ('...')",
+                    NO_PARAM, NO_PARAM, NO_PARAM);
+            }
+        } else {
+            __builtin_unreachable();
+        }
+    }
+
+    if (IRRT_DEBUG_ASSERT) {
+        if (expected_dst_ndims != dst_ndarray->ndims) {
+            raise_exception(
+                SizeT, EXN_ASSERTION_ERROR,
+                "IRRT expected_dst_ndims is {0}, but dst_ndarray->ndims is {1}",
+                expected_dst_ndims, dst_ndarray->ndims, NO_PARAM);
+        }
+    }
+
+    if (src_ndarray->ndims - num_indexed < 0) {
+        raise_exception(SizeT, EXN_INDEX_ERROR,
+                        "too many indices for array: array is {0}-dimensional, "
+                        "but {1} were indexed",
+                        src_ndarray->ndims, num_indexes, NO_PARAM);
+    }
 
     dst_ndarray->data = src_ndarray->data;
     dst_ndarray->itemsize = src_ndarray->itemsize;
@@ -188,7 +231,7 @@ void index(SizeT num_indexes, const NDIndex* indexes,
             dst_axis++;
         } else if (index->type == ND_INDEX_TYPE_ELLIPSIS) {
             // The number of ':' entries this '...' implies.
-            SizeT ellipsis_size = src_ndarray->ndims - (num_indexes - 1);
+            SizeT ellipsis_size = src_ndarray->ndims - num_indexed;
 
             for (SizeT j = 0; j < ellipsis_size; j++) {
                 dst_ndarray->strides[dst_axis] = src_ndarray->strides[src_axis];
@@ -206,6 +249,19 @@ void index(SizeT num_indexes, const NDIndex* indexes,
         dst_ndarray->shape[dst_axis] = src_ndarray->shape[src_axis];
         dst_ndarray->strides[dst_axis] = src_ndarray->strides[src_axis];
     }
+
+    if (IRRT_DEBUG_ASSERT) {
+        if (dst_ndarray->ndims != dst_axis) {
+            raise_exception(SizeT, EXN_ASSERTION_ERROR,
+                            "IRRT dst_ndarray->ndims ({0}) != dst_axis ({1})",
+                            dst_ndarray->ndims, dst_axis, NO_PARAM);
+        }
+        if (src_ndarray->ndims != src_axis) {
+            raise_exception(SizeT, EXN_ASSERTION_ERROR,
+                            "IRRT src_ndarray->ndims ({0}) != src_axis ({1})",
+                            src_ndarray->ndims, src_axis, NO_PARAM);
+        }
+    }
 }
 }  // namespace indexing
 }  // namespace ndarray
diff --git a/nac3core/irrt/irrt/ndarray/reshape.hpp b/nac3core/irrt/irrt/ndarray/reshape.hpp
index 1e32c324..8947a0c3 100644
--- a/nac3core/irrt/irrt/ndarray/reshape.hpp
+++ b/nac3core/irrt/irrt/ndarray/reshape.hpp
@@ -66,7 +66,7 @@ void resolve_and_check_new_shape(SizeT size, SizeT new_ndims,
     bool can_reshape;
     if (neg1_exists) {
         // Let `x` be the unknown dimension
-        // solve `x * <new_size> = <size>`
+        // Solve `x * <new_size> = <size>`
         if (new_size == 0 && size == 0) {
             // `x` has infinitely many solutions
             can_reshape = false;
diff --git a/nac3core/irrt/irrt_everything.hpp b/nac3core/irrt/irrt_everything.hpp
index 707f1af7..73940b52 100644
--- a/nac3core/irrt/irrt_everything.hpp
+++ b/nac3core/irrt/irrt_everything.hpp
@@ -1,5 +1,9 @@
 #pragma once
 
+#ifndef IRRT_DEBUG_ASSERT
+#error IRRT_DEBUG_ASSERT flag is missing!! Please define it to 'false' or 'true'.
+#endif
+
 #include <irrt/core.hpp>
 #include <irrt/exception.hpp>
 #include <irrt/int_defs.hpp>
diff --git a/nac3core/src/codegen/expr.rs b/nac3core/src/codegen/expr.rs
index 0480e96b..6dc827ab 100644
--- a/nac3core/src/codegen/expr.rs
+++ b/nac3core/src/codegen/expr.rs
@@ -2942,3 +2942,43 @@ pub fn gen_expr<'ctx, G: CodeGenerator>(
         _ => unimplemented!(),
     }))
 }
+
+/// Generate LLVM IR for an [`ExprKind::Slice`]
+pub fn gen_slice<'ctx, G: CodeGenerator>(
+    generator: &mut G,
+    ctx: &mut CodeGenContext<'ctx, '_>,
+    lower: &Option<Box<Expr<Option<Type>>>>,
+    upper: &Option<Box<Expr<Option<Type>>>>,
+    step: &Option<Box<Expr<Option<Type>>>>,
+) -> Result<
+    (
+        Option<Instance<'ctx, IntModel<Int32>>>,
+        Option<Instance<'ctx, IntModel<Int32>>>,
+        Option<Instance<'ctx, IntModel<Int32>>>,
+    ),
+    String,
+> {
+    let i32_model = IntModel(Int32); // TODO: Switch to usize
+
+    let mut help = |value_expr: &Option<Box<Expr<Option<Type>>>>| -> Result<_, String> {
+        Ok(match value_expr {
+            None => None,
+            Some(value_expr) => {
+                let value_expr = generator
+                    .gen_expr(ctx, value_expr)?
+                    .unwrap()
+                    .to_basic_value_enum(ctx, generator, ctx.primitives.int32)?;
+
+                let value_expr = i32_model.check_value(generator, ctx.ctx, value_expr).unwrap();
+
+                Some(value_expr)
+            }
+        })
+    };
+
+    let lower = help(lower)?;
+    let upper = help(upper)?;
+    let step = help(step)?;
+
+    Ok((lower, upper, step))
+}
diff --git a/nac3core/src/codegen/model/core.rs b/nac3core/src/codegen/model/core.rs
index 8e47cf02..1afe80f2 100644
--- a/nac3core/src/codegen/model/core.rs
+++ b/nac3core/src/codegen/model/core.rs
@@ -21,11 +21,7 @@ pub trait Model<'ctx>: fmt::Debug + Clone + Copy {
     type Type: BasicType<'ctx>;
 
     /// Return the [`BasicType`] of this model.
-    fn get_type<G: CodeGenerator + ?Sized>(
-        &self,
-        generator: &G,
-        ctx: &'ctx Context,
-    ) -> Self::Type;
+    fn get_type<G: CodeGenerator + ?Sized>(&self, generator: &G, ctx: &'ctx Context) -> Self::Type;
 
     /// Check if a [`BasicType`] is the same type of this model.
     fn check_type<T: BasicType<'ctx>, G: CodeGenerator + ?Sized>(
diff --git a/nac3core/src/codegen/model/int.rs b/nac3core/src/codegen/model/int.rs
index 9f0d9f5f..cf51f673 100644
--- a/nac3core/src/codegen/model/int.rs
+++ b/nac3core/src/codegen/model/int.rs
@@ -97,11 +97,7 @@ impl<'ctx, N: IntKind<'ctx>> Model<'ctx> for IntModel<N> {
     type Type = IntType<'ctx>;
 
     #[must_use]
-    fn get_type<G: CodeGenerator + ?Sized>(
-        &self,
-        generator: &G,
-        ctx: &'ctx Context,
-    ) -> Self::Type {
+    fn get_type<G: CodeGenerator + ?Sized>(&self, generator: &G, ctx: &'ctx Context) -> Self::Type {
         self.0.get_int_type(generator, ctx)
     }
 
diff --git a/nac3core/src/codegen/model/ptr.rs b/nac3core/src/codegen/model/ptr.rs
index fb2d48a2..cc810e13 100644
--- a/nac3core/src/codegen/model/ptr.rs
+++ b/nac3core/src/codegen/model/ptr.rs
@@ -17,11 +17,7 @@ impl<'ctx, Element: Model<'ctx>> Model<'ctx> for PtrModel<Element> {
     type Value = PointerValue<'ctx>;
     type Type = PointerType<'ctx>;
 
-    fn get_type<G: CodeGenerator + ?Sized>(
-        &self,
-        generator: &G,
-        ctx: &'ctx Context,
-    ) -> Self::Type {
+    fn get_type<G: CodeGenerator + ?Sized>(&self, generator: &G, ctx: &'ctx Context) -> Self::Type {
         self.0.get_type(generator, ctx).ptr_type(AddressSpace::default())
     }
 
diff --git a/nac3core/src/codegen/model/structure.rs b/nac3core/src/codegen/model/structure.rs
index cd4b9995..ddc4f8a1 100644
--- a/nac3core/src/codegen/model/structure.rs
+++ b/nac3core/src/codegen/model/structure.rs
@@ -111,11 +111,7 @@ impl<'ctx, S: StructKind<'ctx>> Model<'ctx> for StructModel<S> {
     type Value = StructValue<'ctx>;
     type Type = StructType<'ctx>;
 
-    fn get_type<G: CodeGenerator + ?Sized>(
-        &self,
-        generator: &G,
-        ctx: &'ctx Context,
-    ) -> Self::Type {
+    fn get_type<G: CodeGenerator + ?Sized>(&self, generator: &G, ctx: &'ctx Context) -> Self::Type {
         self.0.get_struct_type(generator, ctx)
     }
 
diff --git a/nac3core/src/codegen/numpy_new.rs b/nac3core/src/codegen/numpy_new.rs
index 0f3df368..6faf6f49 100644
--- a/nac3core/src/codegen/numpy_new.rs
+++ b/nac3core/src/codegen/numpy_new.rs
@@ -4,14 +4,11 @@ use inkwell::values::{BasicValue, BasicValueEnum};
 use nac3parser::ast::StrRef;
 
 use crate::{
-    codegen::{
-        structure::{
-            ndarray::{
-                scalar::split_scalar_or_ndarray, shape_util::parse_numpy_int_sequence,
-                NDArrayObject,
-            },
-            tuple::TupleObject,
+    codegen::structure::{
+        ndarray::{
+            scalar::split_scalar_or_ndarray, shape_util::parse_numpy_int_sequence, NDArrayObject,
         },
+        tuple::TupleObject,
     },
     symbol_resolver::ValueEnum,
     toplevel::{
diff --git a/nac3core/src/codegen/structure/ndarray/indexing.rs b/nac3core/src/codegen/structure/ndarray/indexing.rs
index 5d767140..16888351 100644
--- a/nac3core/src/codegen/structure/ndarray/indexing.rs
+++ b/nac3core/src/codegen/structure/ndarray/indexing.rs
@@ -258,11 +258,11 @@ impl<'ctx> NDArrayObject<'ctx> {
 
 pub mod util {
     use itertools::Itertools;
-    use nac3parser::ast::{Expr, ExprKind};
+    use nac3parser::ast::{Constant, Expr, ExprKind};
 
     use crate::{
-        codegen::{model::*, CodeGenContext, CodeGenerator},
-        typecheck::typedef::Type,
+        codegen::{expr::gen_slice, model::*, CodeGenContext, CodeGenerator},
+        typecheck::typedef::{Type, TypeEnum},
     };
 
     use super::{RustNDIndex, RustUserSlice};
@@ -308,46 +308,37 @@ pub mod util {
         for index_expr in index_exprs {
             // NOTE: Currently nac3core's slices do not have an object representation,
             // so the code/implementation looks awkward - we have to do pattern matching on the expression
-            let ndindex =
-                if let ExprKind::Slice { lower: start, upper: stop, step } = &index_expr.node {
-                    // Helper function here to deduce code duplication
-                    type ValueExpr = Option<Box<Expr<Option<Type>>>>;
-                    let mut help = |value_expr: &ValueExpr| -> Result<_, String> {
-                        Ok(match value_expr {
-                            None => None,
-                            Some(value_expr) => {
-                                let value_expr = generator
-                                    .gen_expr(ctx, value_expr)?
-                                    .unwrap()
-                                    .to_basic_value_enum(ctx, generator, ctx.primitives.int32)?;
+            let ndindex = if let ExprKind::Slice { lower, upper, step } = &index_expr.node {
+                // Handle slices
 
-                                let value_expr =
-                                    i32_model.check_value(generator, ctx.ctx, value_expr).unwrap();
+                // Helper function here to deduce code duplication
+                let (lower, upper, step) = gen_slice(generator, ctx, lower, upper, step)?;
+                RustNDIndex::Slice(RustUserSlice { start: lower, stop: upper, step })
+            } else if let ExprKind::Constant { value: Constant::Ellipsis, .. } = &index_expr.node {
+                // Handle '...'
+                RustNDIndex::Ellipsis
+            } else {
+                match &*ctx.unifier.get_ty(index_expr.custom.unwrap()) {
+                    TypeEnum::TObj { obj_id, .. }
+                        if *obj_id == ctx.primitives.option.obj_id(&ctx.unifier).unwrap() =>
+                    {
+                        // Handle `np.newaxis` / `None`
+                        RustNDIndex::NewAxis
+                    }
+                    _ => {
+                        // Treat and handle everything else as a single element index.
+                        let index =
+                            generator.gen_expr(ctx, index_expr)?.unwrap().to_basic_value_enum(
+                                ctx,
+                                generator,
+                                ctx.primitives.int32, // Must be int32, this checks for illegal values
+                            )?;
+                        let index = i32_model.check_value(generator, ctx.ctx, index).unwrap();
 
-                                Some(value_expr)
-                            }
-                        })
-                    };
-
-                    let start = help(start)?;
-                    let stop = help(stop)?;
-                    let step = help(step)?;
-
-                    RustNDIndex::Slice(RustUserSlice { start, stop, step })
-                } else {
-                    todo!("implement me");
-
-                    // Anything else that is not a slice (might be illegal values),
-                    // For nac3core, this should be e.g., an int32 constant, an int32 variable, otherwise its an error
-                    let index = generator.gen_expr(ctx, index_expr)?.unwrap().to_basic_value_enum(
-                        ctx,
-                        generator,
-                        ctx.primitives.int32,
-                    )?;
-                    let index = i32_model.check_value(generator, ctx.ctx, index).unwrap();
-
-                    RustNDIndex::SingleElement(index)
-                };
+                        RustNDIndex::SingleElement(index)
+                    }
+                }
+            };
             rust_ndindexes.push(ndindex);
         }
         Ok(rust_ndindexes)
diff --git a/nac3core/src/codegen/structure/ndarray/product.rs b/nac3core/src/codegen/structure/ndarray/product.rs
index 96d337ec..e505d253 100644
--- a/nac3core/src/codegen/structure/ndarray/product.rs
+++ b/nac3core/src/codegen/structure/ndarray/product.rs
@@ -13,8 +13,6 @@ impl<'ctx> NDArrayObject<'ctx> {
         assert!(a.ndims >= 2);
         assert!(b.ndims >= 2);
 
-        
-
         todo!()
     }
 }
diff --git a/nac3core/src/typecheck/typedef/mod.rs b/nac3core/src/typecheck/typedef/mod.rs
index e93656e3..e125ff80 100644
--- a/nac3core/src/typecheck/typedef/mod.rs
+++ b/nac3core/src/typecheck/typedef/mod.rs
@@ -621,7 +621,8 @@ impl Unifier {
     }
 
     pub fn unify_call(
-        &mut self, call: &Call,
+        &mut self,
+        call: &Call,
         b: Type,
         signature: &FunSignature,
     ) -> Result<(), TypeError> {