Ticket #27317: 27317.patch

user.php

@@ -1532 +1532 @@
          * @param string $user_login Username after it has been sanitized.
          */
         $user_login = apply_filters( 'pre_user_login', $user_login );
+        if ( is_wp_error($user_login) ) //Test if filter returned a WP_error object.
+                return $user_login; //Let's return the error.
 
+
         //Remove any non-printable chars from the login string to see if we have ended up with an empty username
         $user_login = trim($user_login);