Index: wp-includes/user.php
===================================================================
--- wp-includes/user.php	(revision 19650)
+++ wp-includes/user.php	(working copy)
@@ -74,6 +74,8 @@
 	if ( is_a($user, 'WP_User') ) { return $user; }
 
 	if ( empty($username) || empty($password) ) {
+		if ( is_a($user, 'WP_Error') ) { return $user; }
+
 		$error = new WP_Error();
 
 		if ( empty($username) )