Index: wp-includes/user.php
===================================================================
--- wp-includes/user.php	(revision 22717)
+++ wp-includes/user.php	(working copy)
@@ -70,6 +70,7 @@
  */
 add_filter('authenticate', 'wp_authenticate_username_password', 20, 3);
 function wp_authenticate_username_password($user, $username, $password) {
+	if ( is_wp_error( $user ) ) { return $user; }
 	if ( is_a($user, 'WP_User') ) { return $user; }
 
 	if ( empty($username) || empty($password) ) {