"An illegal choice has been detected. Please contact the site administrator." with exposed sort fields
I've solved by setting a default value in the form alter callback for views exposed form

function mymod_form_views_exposed_form_alter(&amp;$form, &amp;$form_state) {
  if($form_state['view']-&gt;name == 'yourviewname') {
    $_GET['sort_by'] = 'default_sort_name';
    $form_state['view']-&gt;set_exposed_input($_GET);  
    $form_state['input']['sort_by'] = $_GET['sort_by'];
  }
}