diff --git a/core/modules/simpletest/src/TestBase.php b/core/modules/simpletest/src/TestBase.php
index b4901af..664158d 100644
--- a/core/modules/simpletest/src/TestBase.php
+++ b/core/modules/simpletest/src/TestBase.php
@@ -366,7 +366,7 @@ protected function checkRequirements() {
    *   the name of the source file, 'line' is the line number and 'function'
    *   is the caller function itself.
    */
-  protected function assert($status, $message = '', $group = 'Other', array $caller = NULL) {
+  protected function assert($status, $message = '', $group = 'Other', array $caller) {
     // Convert boolean status to string status.
     if (is_bool($status)) {
       $status = $status ? 'pass' : 'fail';