diff --git a/modules/system/system.module b/modules/system/system.module
index 072850e..01ffb33 100644
--- a/modules/system/system.module
+++ b/modules/system/system.module
@@ -2563,9 +2563,9 @@ function _system_rebuild_theme_data() {
       $themes[$base_theme]->sub_themes[$key] = $themes[$key]->info['name'];
     }
     // Copy the 'owner' and 'engine' over if the top level theme uses a theme
-    // engine.
+    // engine and only if both use the same engine.
     if (isset($themes[$base_key]->owner)) {
-      if (isset($themes[$base_key]->info['engine'])) {
+       if (isset($themes[$base_key]->info['engine']) && $themes[$base_key]->info['engine'] == $themes[$key]->info['engine']) {
         $themes[$key]->info['engine'] = $themes[$base_key]->info['engine'];
         $themes[$key]->owner = $themes[$base_key]->owner;
         $themes[$key]->prefix = $themes[$base_key]->prefix;