Only 'form associated' elements have a potential form owner.

Source/core/html/RadioNodeList.cpp

 /*
  * Copyright (c) 2012 Motorola Mobility, Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 70 matching lines @@
             continue;
         inputElement->setChecked(true);
         return;
     }
 }
 
 bool RadioNodeList::checkElementMatchesRadioNodeListFilter(Element* testElement) const
 {
     ASSERT(testElement->hasTagName(objectTag) || testElement->isFormControlElement());
     if (ownerNode()->hasTagName(formTag)) {
-        HTMLFormElement* formElement = 0;
-        if (testElement->hasTagName(objectTag))
-            formElement = toHTMLObjectElement(testElement)->form();
-        else
-            formElement = toHTMLFormControlElement(testElement)->form();
+        HTMLFormElement* formElement = toHTMLElement(testElement)->formOwner();
         if (!formElement || formElement != ownerNode())
             return false;
     }
 
     return testElement->getIdAttribute() == m_name || testElement->getNameAttribute() == m_name;
 }
 
 bool RadioNodeList::nodeMatches(Element* testElement) const
 {
     if (!testElement->hasTagName(objectTag) && !testElement->isFormControlElement())
         return false;
 
     if (testElement->hasTagName(inputTag) && toHTMLInputElement(testElement)->isImageButton())
         return false;
 
     return checkElementMatchesRadioNodeListFilter(testElement);
 }
 
 } // namspace