Call to undefined function i18n_string_update

i get this error when i save a new regex rule

Fatal error: Call to undefined function i18n_string_update() in modules/contrib/webform_validation/webform_validation.module on line 306
Call Stack
# Time Memory Function Location
1 0.0062 282480 {main}( ) ../index.php:0
2 0.4025 34602952 menu_execute_active_handler( ) ../index.php:33
3 0.4035 34679512 call_user_func_array:{includes/menu.inc:517} ( ) ../menu.inc:517
4 0.4035 34680168 drupal_get_form( ) ../menu.inc:517
5 0.4035 34681576 drupal_build_form( ) ../form.inc:130
6 0.4222 36010608 drupal_process_form( ) ../form.inc:385
7 0.4291 36187360 form_execute_handlers( ) ../form.inc:903
8 0.4291 36190704 webform_validation_manage_rule_submit( ) ../form.inc:1513
9 0.4291 36190792 webform_validation_rule_save( ) ../webform_validation.admin.inc:295
10 0.4394 38736072 module_invoke_all( ) ../webform_validation.module:395
11 0.4428 39081496 call_user_func_array:{/includes/module.inc:895} ( ) ../module.inc:895
12 0.4428 39082056 webform_validation_webform_validation( ) ../module.inc:895

thanks!