diff --git a/dom/base/nsDOMMutationObserver.cpp b/dom/base/nsDOMMutationObserver.cpp
index 0a5a14a9d92e..45d9495cb00b 100644
--- a/dom/base/nsDOMMutationObserver.cpp
+++ b/dom/base/nsDOMMutationObserver.cpp
@@ -685,8 +685,7 @@ nsDOMMutationObserver::Observe(nsINode& aTarget,
     filters.SetCapacity(len);
 
     for (uint32_t i = 0; i < len; ++i) {
-      nsCOMPtr<nsIAtom> a = NS_Atomize(filtersAsString[i]);
-      filters.AppendObject(a);
+      filters.AppendElement(NS_Atomize(filtersAsString[i]));
     }
   }