Index: wp-includes/comment.php
===================================================================
--- wp-includes/comment.php	(revision 21468)
+++ wp-includes/comment.php	(working copy)
@@ -655,7 +655,7 @@
 		$post_author = $wpdb->get_var($wpdb->prepare("SELECT post_author FROM $wpdb->posts WHERE ID = %d LIMIT 1", $comment_post_ID));
 	}
 
-	if ( $user && ( $user_id == $post_author || $user->has_cap('moderate_comments') ) ) {
+	if ( isset( $user ) && ( $user_id == $post_author || $user->has_cap( 'moderate_comments' ) ) ) {
 		// The author and the admins get respect.
 		$approved = 1;
 	 } else {