diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index 77ae05767c..4634075a83 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -849,7 +849,7 @@ function preview_theme() {
 	if ( !current_user_can( 'switch_themes' ) )
 		return;
 
-	$_GET['template'] = preg_replace('|[^a-z0-9_.-]|i', '', $_GET['template']);
+	$_GET['template'] = preg_replace('|[^a-z0-9_.-/]|i', '', $_GET['template']);
 
 	if ( validate_file($_GET['template']) )
 		return;
@@ -857,7 +857,7 @@ function preview_theme() {
 	add_filter('template', create_function('', "return '{$_GET['template']}';") );
 
 	if ( isset($_GET['stylesheet']) ) {
-		$_GET['stylesheet'] = preg_replace('|[^a-z0-9_.-]|i', '', $_GET['stylesheet']);
+		$_GET['stylesheet'] = preg_replace('|[^a-z0-9_.-/]|i', '', $_GET['stylesheet']);
 		if ( validate_file($_GET['stylesheet']) )
 			return;
 		add_filter('stylesheet', create_function('', "return '{$_GET['stylesheet']}';") );