diff -u b/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
--- b/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -2604,6 +2604,7 @@
  *   HMAC and timestamp.
  */
 function drupal_valid_test_ua($new_prefix = NULL) {
+  global $drupal_hash_salt;
   static $test_prefix;
 
   if (isset($new_prefix)) {