Index: trunk/src/wp-includes/comment-template.php
===================================================================
--- trunk/src/wp-includes/comment-template.php	(revision 34363)
+++ trunk/src/wp-includes/comment-template.php	(working copy)
@@ -441,7 +441,7 @@
 	$classes[] = ( empty( $comment->comment_type ) ) ? 'comment' : $comment->comment_type;
 
 	// Add classes for comment authors that are registered users.
-	if ( $comment->user_id > 0 && $user = get_userdata( $comment->user_id ) ) {
+	if ( ! empty( $comment->user_id ) && $user = get_userdata( $comment->user_id ) ) {
 		$classes[] = 'byuser';
 		$classes[] = 'comment-author-' . sanitize_html_class( $user->user_nicename, $comment->user_id );
 		// For comment authors who are the author of the post