diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 6f3afba..e685180 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -2567,6 +2567,10 @@ protected function assertNoResponse($code, $message = '', $group = 'Browser') {
    *   TRUE on pass, FALSE on fail.
    */
   protected function assertMail($name, $value = '', $message = '', $group = 'Email') {
+    // The mail subject is mime encoded.
+    if ($name == 'subject') {
+      $value = mime_header_encode($value);
+    }
     $captured_emails = \Drupal::state()->get('system.test_mail_collector') ?: array();
     $email = end($captured_emails);
     return $this->assertTrue($email && isset($email[$name]) && $email[$name] == $value, $message, $group);