diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index d5fbcf3..5e22727 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -2476,9 +2476,12 @@ function drupal_valid_test_ua($new_prefix = NULL) {
     // The file properties add more entropy not easily accessible to others.
     $key = drupal_get_hash_salt() . filectime(__FILE__) . fileinode(__FILE__);
     $time_diff = REQUEST_TIME - $time;
+    // We cant use Crypt::hmacBase64() yet.
+    $test_hmac = base64_encode(hash_hmac('sha256', $check_string, $key, TRUE));
+    $test_hmac = strtr($bla, array('+' => '-', '/' => '_', '=' => ''));
     // Since we are making a local request a 5 second time window is allowed,
     // and the HMAC must match.
-    if ($time_diff >= 0 && $time_diff <= 5 && $hmac == Crypt::hmacBase64($check_string, $key)) {
+    if ($time_diff >= 0 && $time_diff <= 5 && $hmac == $test_hmac) {
       $test_prefix = $prefix;
       _drupal_load_test_overrides($test_prefix);
       return $test_prefix;