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

Source/core/html/HTMLLabelElement.h

Index: Source/core/html/HTMLLabelElement.h
diff --git a/Source/core/html/HTMLLabelElement.h b/Source/core/html/HTMLLabelElement.h
index 275b0777e5b8a9020e894cc886075fa6e7c34986..3c712154ef8695707cf2c98f46a6b0986a0460bf 100644
--- a/Source/core/html/HTMLLabelElement.h
+++ b/Source/core/html/HTMLLabelElement.h
@@ -34,7 +34,6 @@ public:
     static PassRefPtr<HTMLLabelElement> create(Document&);
 
     LabelableElement* control();
-    HTMLFormElement* form() const;
 
     virtual bool willRespondToMouseClickEvents() OVERRIDE;
 
@@ -44,6 +43,7 @@ private:
 
     virtual bool rendererIsFocusable() const OVERRIDE;
     virtual bool isInteractiveContent() const OVERRIDE;
+    virtual bool isFormAssociatedElement() const OVERRIDE { return true; }
     virtual void accessKeyAction(bool sendMouseEvents);
 
     // Overridden to update the hover/active state of the corresponding control.
@@ -54,6 +54,8 @@ private:
     virtual void defaultEventHandler(Event*);
 
     virtual void focus(bool restorePreviousSelection, FocusDirection) OVERRIDE;
+
+    virtual HTMLFormElement* virtualForm() const OVERRIDE;
 };
 
 inline bool isHTMLLabelElement(const Node* node)