diff --git a/core/includes/file.inc b/core/includes/file.inc
index fd77536..5d4a2bc 100644
--- a/core/includes/file.inc
+++ b/core/includes/file.inc
@@ -1441,7 +1441,7 @@ function file_scan_directory($dir, $mask, $options = array(), $depth = 0) {
   // Normalize $dir only once.
   if ($depth == 0) {
     $dir = file_stream_wrapper_uri_normalize($dir);
-    $dir_has_slash = ($dir[drupal_strlen($dir) - 1] === '/');
+    $dir_has_slash = (substr($dir, -1) === '/');
   }
 
   $options['key'] = in_array($options['key'], array('uri', 'filename', 'name')) ? $options['key'] : 'uri';