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

Source/core/html/HTMLLabelElement.cpp

Index: Source/core/html/HTMLLabelElement.cpp
diff --git a/Source/core/html/HTMLLabelElement.cpp b/Source/core/html/HTMLLabelElement.cpp
index 67cf6196d4001df0191e392cae2a91a53d5b69c6..5ea917f0cd4a005ef21d70c09c90b6241e4b5445 100644
--- a/Source/core/html/HTMLLabelElement.cpp
+++ b/Source/core/html/HTMLLabelElement.cpp
@@ -85,7 +85,7 @@ LabelableElement* HTMLLabelElement::control()
     return 0;
 }
 
-HTMLFormElement* HTMLLabelElement::form() const
+HTMLFormElement* HTMLLabelElement::virtualForm() const
 {
     return FormAssociatedElement::findAssociatedForm(this, 0);
 }