diff --git a/wp-includes/feed-rss2-comments.php b/wp-includes/feed-rss2-comments.php
index ac340b526..541dd0eac 100644
--- a/wp-includes/feed-rss2-comments.php
+++ b/wp-includes/feed-rss2-comments.php
@@ -14,6 +14,7 @@ echo '<?xml version="1.0" encoding="'.get_option('blog_charset').'"?'.'>';
 	xmlns:dc="http://purl.org/dc/elements/1.1/"
 	xmlns:atom="http://www.w3.org/2005/Atom"
 	xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
+	<?php do_action('rss2_comments_ns'); ?>
 	>
 <channel>
 	<title><?php