diff --git modules/system/system.api.php modules/system/system.api.php
index c68d893..dda3c73 100644
--- modules/system/system.api.php
+++ modules/system/system.api.php
@@ -2996,6 +2996,9 @@ function hook_install() {
  * See the batch operations page for more information on how to use the batch API:
  * @link http://drupal.org/node/180528 http://drupal.org/node/180528 @endlink
  *
+ * @param $sandbox
+ *   See above.
+ *
  * @throws DrupalUpdateException, PDOException
  *   In case of error, update hooks should throw an instance of DrupalUpdateException
  *   with a meaningful message for the user. If a database query fails for whatever