diff --git a/lib/javascript-static.js b/lib/javascript-static.js
index d4477a8c3c1..219a368f99e 100644
--- a/lib/javascript-static.js
+++ b/lib/javascript-static.js
@@ -298,7 +298,8 @@ function showAdvancedOnClick(button, hidetext, showtext){
         button.form.elements['mform_showadvanced_last'].value = '0';
     }
     var formelements = button.form.elements;
-    for (var i in formelements){
+    // Fixed MDL-10506
+    for (var i = 0; i < formelements.length; i++){
         if (formelements[i] && formelements[i].name && (formelements[i].name=='mform_showadvanced')){
             formelements[i].value = buttontext;
         }