diff --git a/components/script/dom/htmlformelement.rs b/components/script/dom/htmlformelement.rs
index a4cb42711d7..8759fbafe5e 100644
--- a/components/script/dom/htmlformelement.rs
+++ b/components/script/dom/htmlformelement.rs
@@ -203,7 +203,8 @@ impl HTMLFormElementMethods for HTMLFormElement {
                                 elem.downcast::<HTMLTextAreaElement>().unwrap().form_owner()
                             }
                             _ => {
-                                debug_assert!(!elem.downcast::<HTMLElement>().unwrap().is_listed_element());
+                                debug_assert!(!elem.downcast::<HTMLElement>().unwrap().is_listed_element() ||
+                                              elem.local_name() == &atom!("keygen"));
                                 return false;
                             }
                         }