diff --git a/tests/native.test.ts b/tests/native.test.ts
index 01c8a20..7d0a4f1 100644
--- a/tests/native.test.ts
+++ b/tests/native.test.ts
@@ -1,7 +1,7 @@
 import { test, expect } from "bun:test"
 import { VM } from "#vm"
 import { OpCode } from "#opcode"
-import { toValue, toNumber } from "#value"
+import { toValue, toNumber, toString } from "#value"
 
 test("CALL_NATIVE - basic function call", async () => {
   const vm = new VM({
@@ -41,8 +41,8 @@ test("CALL_NATIVE - function with string manipulation", async () => {
   })
 
   vm.registerFunction('concat', (a, b) => {
-    const aStr = a.type === 'string' ? a.value : String(a.value)
-    const bStr = b.type === 'string' ? b.value : String(b.value)
+    const aStr = a.type === 'string' ? a.value : toString(a)
+    const bStr = b.type === 'string' ? b.value : toString(b)
     return toValue(aStr + ' ' + bStr)
   })