diff --git a/wp-includes/comment-template.php b/wp-includes/comment-template.php
index ed41bb6f12..9558ba9f11 100644
--- a/wp-includes/comment-template.php
+++ b/wp-includes/comment-template.php
@@ -239,7 +239,7 @@ function trackback_url( $display = true ) {
 
 function trackback_rdf($timezone = 0) {
 	global $id;
-	if (strpos($_SERVER['HTTP_USER_AGENT'], 'W3C_Validator') !== false) {
+	if (stripos($_SERVER['HTTP_USER_AGENT'], 'W3C_Validator') === false) {
 		echo '<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
 				xmlns:dc="http://purl.org/dc/elements/1.1/"
 				xmlns:trackback="http://madskills.com/public/xml/rss/module/trackback/">