Index: user.php
===================================================================
--- user.php	(revision 27299)
+++ user.php	(working copy)
@@ -1532,7 +1532,10 @@
 	 * @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);