diff --git a/mozilla/xpcom/glue/nsISupportsImpl.cpp b/mozilla/xpcom/glue/nsISupportsImpl.cpp
index f4c77589e03..4a81ece576a 100644
--- a/mozilla/xpcom/glue/nsISupportsImpl.cpp
+++ b/mozilla/xpcom/glue/nsISupportsImpl.cpp
@@ -54,5 +54,6 @@ NS_TableDrivenQI(void* aThis, const QITableEntry* entries,
     ++entries;
   }
 
+  *aInstancePtr = nsnull;
   return NS_ERROR_NO_INTERFACE;
 }