diff --git a/dom/bindings/BindingUtils.h b/dom/bindings/BindingUtils.h
index 7a36e5fa096..122e49eed18 100644
--- a/dom/bindings/BindingUtils.h
+++ b/dom/bindings/BindingUtils.h
@@ -70,6 +70,8 @@ template <class T>
 inline T*
 UnwrapDOMObject(JSObject* obj)
 {
+  MOZ_ASSERT(IsDOMClass(JS_GetClass(obj)));
+
   JS::Value val = js::GetReservedSlot(obj, DOM_OBJECT_SLOT);
   // XXXbz/khuey worker code tries to unwrap interface objects (which have
   // nothing here).  That needs to stop.