Index: modules/system/system.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.module,v
retrieving revision 1.986
diff -u -p -r1.986 system.module
--- modules/system/system.module	31 Oct 2010 22:02:20 -0000	1.986
+++ modules/system/system.module	6 Nov 2010 18:57:59 -0000
@@ -2410,6 +2410,7 @@ function _system_rebuild_theme_data() {
 
   // Set defaults for theme info.
   $defaults = array(
+    'engine' => 'phptemplate',
     'regions' => array(
       'sidebar_first' => 'Left sidebar',
       'sidebar_second' => 'Right sidebar',
@@ -2443,7 +2444,7 @@ function _system_rebuild_theme_data() {
     if (!empty($themes[$key]->info['base theme'])) {
       $sub_themes[] = $key;
     }
-    if (empty($themes[$key]->info['engine'])) {
+    if ($themes[$key]->info['engine'] == 'theme') {
       $filename = dirname($themes[$key]->uri) . '/' . $themes[$key]->name . '.theme';
       if (file_exists($filename)) {
         $themes[$key]->owner = $filename;