diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 42de880..dd750ea 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -2366,12 +2366,6 @@ function drupal_container(ContainerBuilder $reset = NULL) {
  */
 function drupal_valid_test_ua() {
   global $drupal_hash_salt;
-  // No reason to reset this.
-  static $test_prefix;
-
-  if (isset($test_prefix)) {
-    return $test_prefix;
-  }
 
   if (isset($_SERVER['HTTP_USER_AGENT']) && preg_match("/^(simpletest\d+);(.+);(.+);(.+)$/", $_SERVER['HTTP_USER_AGENT'], $matches)) {
     list(, $prefix, $time, $salt, $hmac) = $matches;
@@ -2384,13 +2378,11 @@ function drupal_valid_test_ua() {
     // Since we are making a local request a 5 second time window is allowed,
     // and the HMAC must match.
     if ($time_diff >= 0 && $time_diff <= 5 && $hmac == drupal_hmac_base64($check_string, $key)) {
-      $test_prefix = $prefix;
-      return $test_prefix;
+      return $prefix;
     }
   }
 
-  $test_prefix = FALSE;
-  return $test_prefix;
+  return FALSE;
 }
 
 /**