Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 15155)
+++ wp-includes/ms-functions.php	(working copy)
@@ -731,7 +731,14 @@
 	$meta = unserialize($signup->meta);
 	$user_login = $wpdb->escape($signup->user_login);
 	$user_email = $wpdb->escape($signup->user_email);
-	wpmu_validate_user_signup($user_login, $user_email);
+	$result = wpmu_validate_user_signup($user_login, $user_email);
+	if ( empty( $result['errors']->errors ) ) {
+		$user_login = $result['user_name'];
+		$user_email = $result['user_email'];
+	} else {
+		return $result['errors'];
+	}
+
 	$password = wp_generate_password();
 
 	$user_id = username_exists($user_login);