diff --git a/wp-includes/feed-atom-comments.php b/wp-includes/feed-atom-comments.php
index 1a27bde90..78b4bf150 100644
--- a/wp-includes/feed-atom-comments.php
+++ b/wp-includes/feed-atom-comments.php
@@ -11,6 +11,7 @@ echo '<?xml version="1.0" encoding="' . get_option('blog_charset') . '" ?' . '>'
 <feed
 	xmlns="http://www.w3.org/2005/Atom"
 	xml:lang="<?php echo get_option('rss_language'); ?>"
+	xmlns:thr="http://purl.org/syndication/thread/1.0"
 	<?php do_action('atom_ns'); ?>
 >
 	<title type="text"><?php