diff --git a/core/modules/user/src/Tests/UserValidationTest.php b/core/modules/user/src/Tests/UserValidationTest.php
index 81a9e48..9d68578 100644
--- a/core/modules/user/src/Tests/UserValidationTest.php
+++ b/core/modules/user/src/Tests/UserValidationTest.php
@@ -53,6 +53,7 @@ function testUsernames() {
       'foo@example.com'        => array('Valid username', 'assertNull'),
       'foo@-example.com'       => array('Valid username', 'assertNull'), // invalid domains are allowed in usernames
       'þòøÇߪř€'               => array('Valid username', 'assertNull'),
+      'foo+bar'                => array('Valid username', 'assertNull'), // '+' symbol is allowed
       'ᚠᛇᚻ᛫ᛒᛦᚦ'                => array('Valid UTF8 username', 'assertNull'), // runes
       ' foo'                   => array('Invalid username that starts with a space', 'assertNotNull'),
       'foo '                   => array('Invalid username that ends with a space', 'assertNotNull'),