diff --git a/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php b/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
index 1cf46fc..d5def94 100644
--- a/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
+++ b/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
@@ -241,7 +241,7 @@ protected function assertFieldByName($name, $value = NULL) {
    *   Use $this->assertSession()->fieldNotExists() or
    *   $this->assertSession()->fieldValueNotEquals() instead.
    */
-  protected function assertNoFieldByName($name, $value = NULL) {
+  protected function assertNoFieldByName($name, $value = '') {
     $this->assertSession()->fieldNotExists($name);
     if ($value !== NULL) {
       $this->assertSession()->fieldValueNotEquals($name, (string) $value);