diff --git a/wp-includes/comment-template.php b/wp-includes/comment-template.php
index 97e2c2ba3d..b87844b601 100644
--- a/wp-includes/comment-template.php
+++ b/wp-includes/comment-template.php
@@ -941,11 +941,13 @@ function trackback_url( $deprecated_echo = true ) {
  * @param int $deprecated Not used (Was $timezone = 0).
  */
 function trackback_rdf( $deprecated = '' ) {
-	if ( !empty( $deprecated ) )
+	if ( ! empty( $deprecated ) ) {
 		_deprecated_argument( __FUNCTION__, '2.5' );
+	}
 
-	if ( false !== stripos($_SERVER['HTTP_USER_AGENT'], 'W3C_Validator') )
+	if ( isset( $_SERVER['HTTP_USER_AGENT'] ) && false !== stripos( $_SERVER['HTTP_USER_AGENT'], 'W3C_Validator' ) ) {
 		return;
+	}
 
 	echo '<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
 			xmlns:dc="http://purl.org/dc/elements/1.1/"