diff --git a/core/tests/Drupal/Tests/Core/Mail/MailManagerTest.php b/core/tests/Drupal/Tests/Core/Mail/MailManagerTest.php
index 7ecb9c3b5123ee5190a047b69175ef4cda281c08..fae456308ea1874d7f2387d5107011677ec33621 100644
--- a/core/tests/Drupal/Tests/Core/Mail/MailManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Mail/MailManagerTest.php
@@ -129,7 +129,7 @@ protected function setUpMailManager($interface = []) {
   public function testGetInstance() {
     $interface = [
       'default' => 'php_mail',
-      'default' => 'test_mail_collector',
+      'example_testkey' => 'test_mail_collector',
     ];
     $this->setUpMailManager($interface);