diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 5144fcf..e2134d9 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -12,6 +12,7 @@
 use Drupal\Component\Utility\Crypt;
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Component\Utility\String;
+use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\DependencyInjection\YamlFileLoader;
 use Drupal\Core\DrupalKernel;
@@ -2570,7 +2571,7 @@ protected function assertNoResponse($code, $message = '', $group = 'Browser') {
   protected function assertMail($name, $value = '', $message = '', $group = 'Email') {
     // The mail subject is mime encoded.
     if ($name == 'subject') {
-      $value = mime_header_encode($value);
+      $value = Unicode::mimeHeaderEncode($value);
     }
     $captured_emails = \Drupal::state()->get('system.test_mail_collector') ?: array();
     $email = end($captured_emails);