diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index ad82dd5..6f3afba 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -530,6 +530,12 @@ protected function drupalCreateUser(array $permissions = array(), $name = NULL)
     $edit = array();
     $edit['name']   = !empty($name) ? $name : $this->getRandomGenerator()->string(8, TRUE, array($this, 'randomUsernameValidate'));
     $edit['mail']   = $edit['name'] . '@example.com';
+    // It is possible that name + @example.com is not a valid email address
+    // since user names can contain whitespace and @ characters and start with a
+    // dot.
+    if (!valid_email_address($edit['mail'])) {
+      $edit['mail'] = $this->getRandomGenerator()->name() . '@example.com';
+    }
     $edit['pass']   = user_password();
     $edit['status'] = 1;
     if ($rid) {