Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 5474)
+++ wp-includes/theme.php	(working copy)
@@ -180,6 +180,7 @@
 		$template    = $theme_data['Template'];
 		$stylesheet  = dirname($theme_file);
 
+		$screenshot = false;
 		foreach ( array('png', 'gif', 'jpg', 'jpeg') as $ext ) {
 			if (file_exists("$theme_root/$stylesheet/screenshot.$ext")) {
 				$screenshot = "screenshot.$ext";