diff --git a/core/includes/file.inc b/core/includes/file.inc
index 26dfb64..8f45375 100644
--- a/core/includes/file.inc
+++ b/core/includes/file.inc
@@ -500,7 +500,8 @@ function file_create_url($uri) {
  *   otherwise.
  */
 function file_prepare_directory(&$directory, $options = FILE_MODIFY_PERMISSIONS) {
-  if (!file_stream_wrapper_valid_scheme(file_uri_scheme($directory))) {
+  $scheme = file_uri_scheme($directory);
+  if (!$scheme || !file_stream_wrapper_valid_scheme($scheme)) {
     // Only trim if we're not dealing with a stream.
     $directory = rtrim($directory, '/\\');
   }