diff -u b/core/modules/forum/src/Tests/ForumTest.php b/core/modules/forum/src/Tests/ForumTest.php
--- b/core/modules/forum/src/Tests/ForumTest.php
+++ b/core/modules/forum/src/Tests/ForumTest.php
@@ -488,9 +488,7 @@
     $edit['comment_body[0][value]'] = $this->randomName();
     $this->drupalPostForm('node/' . $node->id(), $edit, t('Save'));
 
-    $urls = $this->xpath('//a[normalize-space()=:label]', array(':label' => 'Reply'));
-
-    $this->drupalPostForm($urls[0]['href'], $edit, t('Save'));
+    $this->drupalPostForm('comment/reply/node/'. $node->id() . '/comment_forum/1', $edit, t('Save'));
     $indented = $this->cssSelect('div .indented .comment');
     $this->assertTrue(empty($indented), 'Check that comment display in flat mode');
   }