--- a/layout/style/Loader.h
+++ b/layout/style/Loader.h
@@ -509,18 +509,19 @@ private:
 
   // The array of posted stylesheet loaded events (SheetLoadDatas) we have.
   // Note that these are rare.
   LoadDataArray     mPostedEvents;
 
   // Our array of "global" observers
   nsTObserverArray<nsCOMPtr<nsICSSLoaderObserver> > mObservers;
 
-  // the load data needs access to the document...
-  nsIDocument*      mDocument;  // the document we live for
+  // This reference is nulled by the Document in it's destructor through
+  // DropDocumentReference().
+  nsIDocument* MOZ_NON_OWNING_REF mDocument;  // the document we live for
 
 
   // Number of datas still waiting to be notified on if we're notifying on a
   // whole bunch at once (e.g. in one of the stop methods).  This is used to
   // make sure that HasPendingLoads() won't return false until we're notifying
   // on the last data we're working with.
   uint32_t          mDatasToNotifyOn;