--- a/js/src/ion/VMFunctions.cpp
+++ b/js/src/ion/VMFunctions.cpp
@@ -359,17 +359,18 @@ ArrayConcatDense(JSContext *cx, HandleOb
     if (!js::array_concat(cx, 1, argv))
         return NULL;
     return &argv[0].toObject();
 }
 
 bool
 CharCodeAt(JSContext *cx, HandleString str, int32_t index, uint32_t *code)
 {
-    JS_ASSERT(index < str->length());
+    JS_ASSERT(index >= 0 &&
+              static_cast<uint32_t>(index) < str->length());
 
     const jschar *chars = str->getChars(cx);
     if (!chars)
         return false;
 
     *code = chars[index];
     return true;
 }