An error occurred at function : drush_core_cache_clear() and drush_core_updatedb() in `drush updatedb`

At the end of drush updatedb I always end up with the following:

An error occurred at function : drush_core_cache_clear      [error]
Finished performing updates.      [ok]
An error occurred at function : drush_core_updatedb    [error]

Everything seems to work fine, but the error is annoying. I though it may be due to me running drupal_flush_all_caches(); in my hook_update, but even attempting to run the following update hook results in the same error:

function mymodule_update_6002() {
  $ret = array();
  return $ret;
}

I have tried with the latest stable release, and with HEAD. same deal.