diff --git a/modules/system/system.module b/modules/system/system.module
index 4868370831..b8d374fc48 100644
--- a/modules/system/system.module
+++ b/modules/system/system.module
@@ -2603,6 +2603,13 @@ function _system_rebuild_theme_data() {
         $themes[$key]->prefix = $engines[$engine]->name;
         $themes[$key]->template = TRUE;
       }
+      else {
+        // Avoid warning when used directly after in _drupal_theme_initialize
+        $themes[$key]->engine = NULL;
+        $themes[$key]->owner = NULL;
+        $themes[$key]->prefix = NULL;
+        $themes[$key]->template = NULL;
+      }
     }
 
     // Prefix stylesheets and scripts with module path.