diff --git a/src/lib/dom-module.html b/src/lib/dom-module.html
index 7fae6b64c4..b3b438a789 100644
--- a/src/lib/dom-module.html
+++ b/src/lib/dom-module.html
@@ -46,6 +46,9 @@
         this.getAttribute('name') || this.getAttribute('is');
       if (id) {
         this.id = id;
+        // store id separate from lowercased id so that
+        // in all cases mixedCase id will stored distinctly
+        // and lowercase version is a fallback
         modules[id] = this;
         lcModules[id.toLowerCase()] = this;
       }
@@ -87,20 +90,15 @@
   // loaded by this point. In addition the HTMLImports polyfill should be
   // changed to upgrade elements prior to running any scripts.)
   var cePolyfill = window.CustomElements && !CustomElements.useNative;
-  if (cePolyfill) {
-    var ready = CustomElements.ready;
-    CustomElements.ready = true;
-  }
   document.registerElement('dom-module', DomModule);
-  if (cePolyfill) {
-    CustomElements.ready = ready;
-  }
 
   function forceDocumentUpgrade() {
     if (cePolyfill) {
       var script = document._currentScript || document.currentScript;
-      if (script) {
-        CustomElements.upgradeAll(script.ownerDocument);
+      var doc = script && script.ownerDocument;
+      if (doc && !doc.__customElementsForceUpgraded) {
+        doc.__customElementsForceUpgraded = true;
+        CustomElements.upgradeAll(doc);
       }
     }
   }