diff --git a/wp-includes/comment-functions.php b/wp-includes/comment-functions.php
index 1bd0860ab..842135ef6 100644
--- a/wp-includes/comment-functions.php
+++ b/wp-includes/comment-functions.php
@@ -94,7 +94,7 @@ function wp_allow_comment($commentdata) {
 	global $wpdb;
 	extract($commentdata);
 
-	$comment_user_domain = apply_filters('pre_comment_user_domain', gethostbyaddr($comment_author_ip) );
+	$comment_user_domain = apply_filters('pre_comment_user_domain', gethostbyaddr($comment_author_IP) );
 
 	// Simple duplicate check
 	$dupe = "SELECT comment_ID FROM $wpdb->comments WHERE comment_post_ID = '$comment_post_ID' AND ( comment_author = '$comment_author' ";