Index: src/wp-admin/includes/comment.php
===================================================================
--- src/wp-admin/includes/comment.php	(revision 31273)
+++ src/wp-admin/includes/comment.php	(working copy)
@@ -37,27 +37,29 @@
 	if ( ! current_user_can( 'edit_comment', (int) $_POST['comment_ID'] ) )
 		wp_die ( __( 'You are not allowed to edit comments on this post.' ) );
 
+	$comment_data = array();
+
 	if ( isset( $_POST['newcomment_author'] ) )
-		$_POST['comment_author'] = $_POST['newcomment_author'];
+		$comment_data['comment_author'] = $_POST['newcomment_author'];
 	if ( isset( $_POST['newcomment_author_email'] ) )
-		$_POST['comment_author_email'] = $_POST['newcomment_author_email'];
+		$comment_data['comment_author_email'] = $_POST['newcomment_author_email'];
 	if ( isset( $_POST['newcomment_author_url'] ) )
-		$_POST['comment_author_url'] = $_POST['newcomment_author_url'];
+		$comment_data['comment_author_url'] = $_POST['newcomment_author_url'];
 	if ( isset( $_POST['comment_status'] ) )
-		$_POST['comment_approved'] = $_POST['comment_status'];
+		$comment_data['comment_approved'] = $_POST['comment_status'];
 	if ( isset( $_POST['content'] ) )
-		$_POST['comment_content'] = $_POST['content'];
+		$comment_data['comment_content'] = $_POST['content'];
 	if ( isset( $_POST['comment_ID'] ) )
-		$_POST['comment_ID'] = (int) $_POST['comment_ID'];
+		$comment_data['comment_ID'] = (int) $_POST['comment_ID'];
 
-	foreach ( array ('aa', 'mm', 'jj', 'hh', 'mn') as $timeunit ) {
-		if ( !empty( $_POST['hidden_' . $timeunit] ) && $_POST['hidden_' . $timeunit] != $_POST[$timeunit] ) {
-			$_POST['edit_date'] = '1';
+	foreach ( array( 'aa', 'mm', 'jj', 'hh', 'mn' ) as $timeunit ) {
+		if ( ! empty( $_POST['hidden_' . $timeunit] ) && $_POST['hidden_' . $timeunit] != $_POST[$timeunit] ) {
+			$comment_data['edit_date'] = '1';
 			break;
 		}
 	}
 
-	if ( !empty ( $_POST['edit_date'] ) ) {
+	if ( ! empty( $comment_data['edit_date'] ) ) {
 		$aa = $_POST['aa'];
 		$mm = $_POST['mm'];
 		$jj = $_POST['jj'];
@@ -68,10 +70,10 @@
 		$hh = ($hh > 23 ) ? $hh -24 : $hh;
 		$mn = ($mn > 59 ) ? $mn -60 : $mn;
 		$ss = ($ss > 59 ) ? $ss -60 : $ss;
-		$_POST['comment_date'] = "$aa-$mm-$jj $hh:$mn:$ss";
+		$comment_data['comment_date'] = "$aa-$mm-$jj $hh:$mn:$ss";
 	}
 
-	wp_update_comment( $_POST );
+	wp_update_comment( $comment_data );
 }
 
 /**
Index: tests/phpunit/tests/comment.php
===================================================================
--- tests/phpunit/tests/comment.php	(revision 31273)
+++ tests/phpunit/tests/comment.php	(working copy)
@@ -36,13 +36,14 @@
 	 * @ticket 30307
 	 */
 	function test_wp_update_comment_updates_user_id() {
+		wp_set_current_user( 1 );
 		$post_id = $this->factory->post->create();
-		$comment_id = $this->factory->comment->create( array( 'comment_post_ID' => $post_id ) );
+		$comment_id = $this->factory->comment->create( array( 'comment_post_ID' => $post_id, 'user_id' => 1 ) );
 
-		wp_update_comment( array( 'comment_ID' => $comment_id, 'user_id' => 1 ) );
+		wp_update_comment( array( 'comment_ID' => $comment_id, 'user_id' => 0 ) );
 
 		$comment = get_comment( $comment_id );
-		$this->assertEquals( 1, $comment->user_id );
+		$this->assertEquals( 0, $comment->user_id );
 	}
 
 	public function test_get_approved_comments() {