--- a/mail/components/shell/nsGNOMEShellService.cpp
+++ b/mail/components/shell/nsGNOMEShellService.cpp
@@ -207,17 +207,20 @@ bool nsGNOMEShellService::checkDefault(c
     if (giovfs) {
       handler.Truncate();
       nsCOMPtr<nsIHandlerApp> handlerApp;
       rv = giovfs->GetAppForURIScheme(nsDependentCString(aProtocols[i]),
                                       getter_AddRefs(handlerApp));
       if (NS_FAILED(rv) || !handlerApp) {
         return false;
       }
-      nsCOMPtr<nsIGIOMimeApp> app = do_QueryInterface(handlerApp);
+      nsCOMPtr<nsIGIOMimeApp> app = do_QueryInterface(handlerApp, &rv);
+      if (NS_FAILED(rv) || !app) {
+        return false;
+      }
       rv = app->GetCommand(handler);
       if (NS_SUCCEEDED(rv) && !CheckHandlerMatchesAppName(handler)) {
         return false;
       }
     }
   }
 
   return true;