diff --git a/src/interpreter/instructions/mod.rs b/src/interpreter/instructions/mod.rs
index 5681620..cf71c6e 100644
--- a/src/interpreter/instructions/mod.rs
+++ b/src/interpreter/instructions/mod.rs
@@ -27,7 +27,7 @@ pub enum Instruction {
     /// The `call-core` instruction.
     CallCore {
         /// The function index.
-        function_index: usize,
+        function_index: u32,
     },
 
     /// The `s8.from_i32` instruction.