protected function processPageId($page_id) {
    $page_ids = array_keys($this->getVisiblePages());

    // Redirect to default page (first page) if no page or invalid page is
    // requested.
    if (empty($page_id) || !in_array($page_id, $page_ids)) {
      $page_id = reset($page_ids);
    }

    return $page_id;
  }