Index: includes/theme.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/theme.inc,v
retrieving revision 1.562
diff -u -p -r1.562 theme.inc
--- includes/theme.inc	4 Jan 2010 16:20:20 -0000	1.562
+++ includes/theme.inc	4 Jan 2010 20:14:43 -0000
@@ -929,7 +929,12 @@ function theme($hook, $variables = array
 
   // restore path_to_theme()
   $theme_path = $temp;
-  return $output;
+  if(isset($output)) {
+  	return $output;
+  }
+  else {
+  	return '';
+  }
 }
 
 /**