Empty response after a callback in form_alter

Question

I am trying to add a new field in a preexistent form and invoke a callback. The response of the callback is alway an error with an empty response:

An AJAX HTTP error occurred.
HTTP Result Code: 200
Debugging information follows.
Path: /system/ajax
StatusText: OK
ResponseText:  

ResponseText is always empty.

The code I use in my_form_form_alter() is the following.

$form['country_views_exposed_form'] = array(
  '#type' => 'select',
  '#title' => t('Country'),
  '#options' => array(
    'sl'  => 'Select...',
    'au'  => 'Australia',
    'nz'  => 'New Zealand'
  ),
  '#ajax' => array(
    'callback' => '__my_callback',
    'wrapper' => 'country-views-exposed-form-wrapper-',
    'method' => 'replace',
    'event' => 'change'
  ),
  '#attributes' => array('class' => 'views-exposed-widget')
);

All I put in __my_callback() produces the same result: I don't get a clean response.

function __my_callback(&$form, &$form_state) { 
  return $form['states_views_exposed_form'];
}

Answer 1

I have resolved. Exposed form fields don't are the same of 'normal' form fields in drupal. I have applied the patch #46 listed here: http://drupal.org/node/1183418