diff --git a/wp-includes/comment-functions.php b/wp-includes/comment-functions.php
index efe4c95827..a61610aea5 100644
--- a/wp-includes/comment-functions.php
+++ b/wp-includes/comment-functions.php
@@ -78,6 +78,8 @@ function wp_insert_comment($commentdata) {
 		$comment_parent = 0;
 	if ( ! isset($comment_approved) )
 		$comment_approved = 1;
+	if ( ! isset($user_id) )
+		$user_id = 0;
 
 	$result = $wpdb->query("INSERT INTO $wpdb->comments 
 	(comment_post_ID, comment_author, comment_author_email, comment_author_url, comment_author_IP, comment_date, comment_date_gmt, comment_content, comment_approved, comment_agent, comment_type, comment_parent, user_id)