diff --git a/dom/base/nsDOMClassInfo.cpp b/dom/base/nsDOMClassInfo.cpp
index 5afc42700a3..7e8c81177a5 100644
--- a/dom/base/nsDOMClassInfo.cpp
+++ b/dom/base/nsDOMClassInfo.cpp
@@ -5509,7 +5509,7 @@ private:
 
   static PRBool IsConstructable(const nsDOMClassInfoData *aData)
   {
-    if (IS_EXTERNAL(aData)) {
+    if (IS_EXTERNAL(aData->mCachedClassInfo)) {
       const nsExternalDOMClassInfoData* data =
         static_cast<const nsExternalDOMClassInfoData*>(aData);
       return data->mConstructorCID != nsnull;