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

Source/core/html/RadioNodeList.cpp

Index: Source/core/html/RadioNodeList.cpp
diff --git a/Source/core/html/RadioNodeList.cpp b/Source/core/html/RadioNodeList.cpp
index 0522f12ec501a590bbf86057b4812471a0730461..f7cbfa982a2f55660b47bc3c18b2d3d583b25664 100644
--- a/Source/core/html/RadioNodeList.cpp
+++ b/Source/core/html/RadioNodeList.cpp
@@ -88,11 +88,7 @@ bool RadioNodeList::checkElementMatchesRadioNodeListFilter(Element* testElement)
 {
     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;
     }