diff --git a/src/decoders/binary.rs b/src/decoders/binary.rs
index f1f7f77..d8699a6 100644
--- a/src/decoders/binary.rs
+++ b/src/decoders/binary.rs
@@ -169,7 +169,7 @@ fn instruction<'input, E: ParseError<&'input [u8]>>(
             (
                 input,
                 Instruction::CallCore {
-                    function_index: argument_0 as usize,
+                    function_index: argument_0 as u32,
                 },
             )
         }
diff --git a/src/decoders/wat.rs b/src/decoders/wat.rs
index 3e41d51..4a77857 100644
--- a/src/decoders/wat.rs
+++ b/src/decoders/wat.rs
@@ -146,7 +146,7 @@ impl<'a> Parse<'a> for Instruction {
             parser.parse::<keyword::call_core>()?;
 
             Ok(Instruction::CallCore {
-                function_index: parser.parse::<u64>()? as usize,
+                function_index: parser.parse::<u32>()?,
             })
         } else if lookahead.peek::<keyword::s8_from_i32>() {
             parser.parse::<keyword::s8_from_i32>()?;
diff --git a/src/interpreter/instruction.rs b/src/interpreter/instruction.rs
index 712a6ea..37f8162 100644
--- a/src/interpreter/instruction.rs
+++ b/src/interpreter/instruction.rs
@@ -12,7 +12,7 @@ pub enum Instruction {
     /// The `call-core` instruction.
     CallCore {
         /// The function index.
-        function_index: usize,
+        function_index: u32,
     },
 
     /// The `s8.from_i32` instruction.
diff --git a/src/interpreter/instructions/call_core.rs b/src/interpreter/instructions/call_core.rs
index 5a3da21..7a8e498 100644
--- a/src/interpreter/instructions/call_core.rs
+++ b/src/interpreter/instructions/call_core.rs
@@ -8,16 +8,16 @@ use crate::{
 };
 
 executable_instruction!(
-    call_core(function_index: usize, instruction: Instruction) -> _ {
+    call_core(function_index: u32, instruction: Instruction) -> _ {
         move |runtime| -> _ {
             let instance = &mut runtime.wasm_instance;
-            let index = FunctionIndex::new(function_index);
+            let index = FunctionIndex::new(function_index as usize);
 
             let local_or_import = instance.local_or_import(index).ok_or_else(|| {
                 InstructionError::new(
                     instruction,
                     InstructionErrorKind::LocalOrImportIsMissing {
-                        function_index: function_index as u32,
+                        function_index: function_index,
                     },
                 )
             })?;
@@ -40,7 +40,7 @@ executable_instruction!(
                 return Err(InstructionError::new(
                     instruction,
                     InstructionErrorKind::LocalOrImportSignatureMismatch {
-                        function_index: function_index as u32,
+                        function_index: function_index,
                         expected: (local_or_import.inputs().to_vec(), vec![]),
                         received: (input_types, vec![]),
                     },
@@ -51,7 +51,7 @@ executable_instruction!(
                 InstructionError::new(
                     instruction,
                     InstructionErrorKind::LocalOrImportCall {
-                        function_index: function_index as u32,
+                        function_index: function_index,
                     },
                 )
             })?;