diff --git a/dom/base/CustomElementRegistry.cpp b/dom/base/CustomElementRegistry.cpp
index 4381d8b34d05..16c0be5e4c53 100644
--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -423,6 +423,11 @@ CustomElementRegistry::EnqueueLifecycleCallback(nsIDocument::ElementCallbackType
         definition->mLocalName != aCustomElement->NodeInfo()->NameAtom()) {
       return;
     }
+
+    if (!definition->mCallbacks) {
+      // definition has been unlinked.  Don't try to mess with it.
+      return;
+    }
   }
 
   auto callback =