diff --git a/lib/6to5/polyfill.js b/lib/6to5/polyfill.js
index 5d945d16e8..9c2cf47317 100644
--- a/lib/6to5/polyfill.js
+++ b/lib/6to5/polyfill.js
@@ -6,7 +6,11 @@ var ensureSymbol = function (key) {
 
 var ensureProto = function (Constructor, key, val) {
   var proto = Constructor.prototype;
-  proto[key] = proto[key] || val;
+  if (!proto[key]) {
+    Object.defineProperty(proto, key, {
+      value: val
+    });
+  }
 };
 
 //