ajax callback on form element checkboxes

I am having some troubles with ajax on form element checkboxes, in ajax callback form_state is without values and triggering element etc .. why is that?

The form is called with drupal_get_form and this is my element

$form['select_options'] = array(
  '#type' => 'checkboxes',
  '#options' => $options, 
  '#ajax' => array(
    'callback' => 'ajax_options_callback',
    'wrapper' => 'options-form',
    'event' => 'change',
    'method' => 'replace',
  ),
);