Exception: Plugin 'reposync' of type 'reposync' does not contain a valid class name in handler slot 'worker'

When I run versioncontrol_git_update_6207() on my d.o testing site, I get this exception:

Exception: Plugin 'reposync' of type 'reposync' does not contain a valid class name in handler slot 'worker' in VersioncontrolRepository->getPluginClass() (line 528 of /var/www/dev/vcs-git-drupal.redesign.devdrupal.org/htdocs/sites/all/modules/versioncontrol/includes/VersioncontrolRepository.php).

The function with some debugging info:

  protected function getPluginClass($plugin_slot, $plugin_type, $class_type) {
    // plugin slot: reposync
    // plugin type: reposync
    // class type: worker

    $plugin = $this->getPlugin($plugin_slot, $plugin_type);
    /*
Array
(
    [git_default] => Array
        (
            [title] => Default Git history synchronizer
            [worker] => Array
                (
                    [class] => VersioncontrolGitRepositoryHistorySynchronizerDefault
                )

            [module] => versioncontrol_git
            [name] => git_default
            [path] => sites/all/modules/versioncontrol_git/includes/plugins/reposync
            [file] => git_default.inc
            [plugin module] => versioncontrol
            [plugin type] => reposync
        )

)
*/

    $class_name = ctools_plugin_get_class($plugin, $class_type); // <-- $class_name was empty
    if (!class_exists($class_name)) {
      throw new Exception("Plugin '$plugin_slot' of type '$plugin_type' does not contain a valid class name in handler slot '$class_type'", E_WARNING);
      return FALSE;
    }

    $plugin_object = new $class_name();
    $this->getBackend()->verifyPluginInterface($this, $plugin_slot, $plugin_object);
    return $plugin_object;
  }

Any idea what's wrong here or how a valid reposync plugin is supposed to get in?