Hierarchical select: Fatal error: Call to undefined function tt()

Fatal error: Call to undefined function tt() in hs_taxonomy.module on line 939

function _hs_taxonomy_hierarchical_select_terms_to_options($terms) {
  $options = array();
  foreach ($terms as $key => $term) {
    // Use the translated term when available!
    if (module_exists('i18n_taxonomy')) {
      $options[$term->tid] = tt("taxonomy:term:$term->tid:name", $term->name);
    }
    else {
      $options[$term->tid] = t($term->name);
    }
  }
  return $options;
}

Wim (matainer of HS) says that this issue is not related to HS.

Can somebody say me please that is wrong?

thank u.

p.d. sorry for my english