Subtheme creation with Drush from outside the installation folder doesn't work

When trying to execute drush fst mytheme from outside the Drupal installation folder (in a Grunt + grunt-drush package task for example) the drush_locate_root() call returns false causing the following error:

Source directory /themes/contrib/zurb_foundation/STARTER/ is not readable or does not exist.

Grunt task example:

      'create-subtheme': {
        args: [
          'fst',
          'mytheme',
          '--root=' + buildProperties.target_dir + '/docroot',
        ]
      }