diff --git a/lib/6to5/browser.js b/lib/6to5/browser.js
index <HASH>..<HASH> 100644
--- a/lib/6to5/browser.js
+++ b/lib/6to5/browser.js
@@ -75,6 +75,6 @@ var runScripts = function () {
 
 if (global.addEventListener) {
   global.addEventListener("DOMContentLoaded", runScripts, false);
-} else {
+} else if (global.attachEvent) {
   global.attachEvent("onload", runScripts);
 }
Check if attachEvent is defined