diff --git a/wp-comments-post.php b/wp-comments-post.php
index 8fa0548991..923b03b99c 100644
--- a/wp-comments-post.php
+++ b/wp-comments-post.php
@@ -75,7 +75,7 @@ if ( '' == $comment_content )
 
 $comment_parent = isset($_POST['comment_parent']) ? absint($_POST['comment_parent']) : 0;
 
-$commentdata = compact('comment_post_ID', 'comment_author', 'comment_author_email', 'comment_author_url', 'comment_content', 'comment_type', 'comment_parent', 'user_id');
+$commentdata = compact('comment_post_ID', 'comment_author', 'comment_author_email', 'comment_author_url', 'comment_content', 'comment_type', 'comment_parent', 'user_ID');
 
 $comment_id = wp_new_comment( $commentdata );
 
diff --git a/wp-includes/comment.php b/wp-includes/comment.php
index cdf6ae249d..4a3615ef6d 100644
--- a/wp-includes/comment.php
+++ b/wp-includes/comment.php
@@ -1126,11 +1126,10 @@ function wp_insert_comment($commentdata) {
  * @return array Parsed comment information.
  */
 function wp_filter_comment($commentdata) {
-	// user_id is preferred. user_ID is accepted for back-compat.
 	if ( isset($commentdata['user_ID']) )
-		$commentdata['user_id'] = $commentdata['user_ID'] = apply_filters('pre_user_id', $commentdata['user_ID']);
-	else
-		$commentdata['user_id'] = $commentdata['user_ID'] = apply_filters('pre_user_id', $commentdata['user_id']);
+		$commentdata['user_id'] = apply_filters('pre_user_id', $commentdata['user_ID']);
+	elseif ( isset($commentdata['user_id']) )
+		$commentdata['user_id'] = apply_filters('pre_user_id', $commentdata['user_id']);
 	$commentdata['comment_agent']        = apply_filters('pre_comment_user_agent', $commentdata['comment_agent']);
 	$commentdata['comment_author']       = apply_filters('pre_comment_author_name', $commentdata['comment_author']);
 	$commentdata['comment_content']      = apply_filters('pre_comment_content', $commentdata['comment_content']);
@@ -1181,11 +1180,10 @@ function wp_new_comment( $commentdata ) {
 	$commentdata = apply_filters('preprocess_comment', $commentdata);
 
 	$commentdata['comment_post_ID'] = (int) $commentdata['comment_post_ID'];
-	// user_id is preferred. user_ID is accepted for back-compat.
 	if ( isset($commentdata['user_ID']) )
 		$commentdata['user_id'] = $commentdata['user_ID'] = (int) $commentdata['user_ID'];
-	else
-		$commentdata['user_id'] = $commentdata['user_ID'] = (int) $commentdata['user_id'];
+	elseif ( isset($commentdata['user_id']) )
+		$commentdata['user_id'] = (int) $commentdata['user_id'];
 
 	$commentdata['comment_parent'] = isset($commentdata['comment_parent']) ? absint($commentdata['comment_parent']) : 0;
 	$parent_status = ( 0 < $commentdata['comment_parent'] ) ? wp_get_comment_status($commentdata['comment_parent']) : '';
diff --git a/wp-includes/user.php b/wp-includes/user.php
index 120ec65ada..749558895d 100644
--- a/wp-includes/user.php
+++ b/wp-includes/user.php
@@ -451,7 +451,7 @@ function update_usermeta( $user_id, $meta_key, $meta_value ) {
  * @param int $for_user_id Optional. User ID to setup global data.
  */
 function setup_userdata($for_user_id = '') {
-	global $user_login, $userdata, $user_level, $user_id, $user_ID, $user_email, $user_url, $user_pass_md5, $user_identity;
+	global $user_login, $userdata, $user_level, $user_ID, $user_email, $user_url, $user_pass_md5, $user_identity;
 
 	if ( '' == $for_user_id )
 		$user = wp_get_current_user();
@@ -464,7 +464,7 @@ function setup_userdata($for_user_id = '') {
 	$userdata = $user->data;
 	$user_login	= $user->user_login;
 	$user_level	= (int) isset($user->user_level) ? $user->user_level : 0;
-	$user_id = $user_ID = (int) $user->ID;
+	$user_ID = (int) $user->ID;
 	$user_email	= $user->user_email;
 	$user_url	= $user->user_url;
 	$user_pass_md5	= md5($user->user_pass);