diff --git a/lib/6to5/transformation/templates/extends.js b/lib/6to5/transformation/templates/extends.js
index 89cff6502b..f8276831b1 100644
--- a/lib/6to5/transformation/templates/extends.js
+++ b/lib/6to5/transformation/templates/extends.js
@@ -1,4 +1,4 @@
-(function (target) {
+Object.assign || (function (target) {
   for (var i = 1; i < arguments.length; i++) {
     var source = arguments[i];
     for (var key in source) {