diff --git a/js/src/jsfun.c b/js/src/jsfun.c
index 0d3a9fd2628e..01ad3a4c6853 100644
--- a/js/src/jsfun.c
+++ b/js/src/jsfun.c
@@ -394,8 +394,11 @@ args_setProperty(JSContext *cx, JSObject *obj, jsval id, jsval *vp)
         break;
 
       default:
-        if ((uintN)slot < fp->argc && !ArgWasDeleted(cx, fp, slot))
+        if (fp->fun->interpreted &&
+            (uintN)slot < fp->argc &&
+            !ArgWasDeleted(cx, fp, slot)) {
             fp->argv[slot] = *vp;
+        }
         break;
     }
     return JS_TRUE;