diff -u b/core/modules/language/lib/Drupal/language/Tests/LanguageUrlRewritingTest.php b/core/modules/language/lib/Drupal/language/Tests/LanguageUrlRewritingTest.php
--- b/core/modules/language/lib/Drupal/language/Tests/LanguageUrlRewritingTest.php
+++ b/core/modules/language/lib/Drupal/language/Tests/LanguageUrlRewritingTest.php
@@ -101,7 +101,7 @@
   function testDomainNameNegotiationPort() {
     $language_domain = 'example.fr';
     $edit = array(
-      'language_negotiation_url_part' => 1,
+      'language_negotiation_url_part' => LANGUAGE_NEGOTIATION_URL_DOMAIN,
       'domain[fr]' => $language_domain
     );
     $this->drupalPost('admin/config/regional/language/detection/url', $edit, t('Save configuration'));