Comment valider un textfield generer par #ajax

<?php
/<strong>
 * Show/hide textfields based on AJAX-enabled checkbox clicks.
 */
function ajax_example_autotextfields($form, &$form_state) {

  $form['ask_first_name'] = array(
    '#type' => 'checkbox',
    '#title' => t('Ask me my first name'),
    '#ajax' => array(
      'callback' => 'ajax_example_autotextfields_callback',
      'wrapper' => 'textfields',
      'effect' => 'fade',
    )
  );
  $form['ask_last_name'] = array(
   '#type' => 'checkbox',
   '#title' => t('Ask me my last name'),
    '#ajax' => array(
      'callback' => 'ajax_example_autotextfields_callback',
      'wrapper' => 'textfields',
      'effect' => 'fade',
    ),
  );

  $form['textfields'] = array(
    '#title' => t("Generated text fields for first and last name"),
    '#prefix' => '<div id="textfields">',
    '#suffix' => '</div>',
    '#type' => 'fieldset',
    '#description' => t('This is where we put automatically generated textfields'),
  );

  // Since checkboxes return TRUE or FALSE, we have to check that
  // $form_state has been filled as well as what it contains.
  if (!empty($form_state['values']['ask_first_name']) && $form_state['values']['ask_first_name']) {
    $form['textfields']['first_name'] = array(
      '#type' => 'textfield',
      '#title' => t('First Name'),
    );
  }
  if (!empty($form_state['values']['ask_last_name']) && $form_state['values']['ask_last_name']) {
    $form['textfields']['last_name'] = array(
      '#type' => 'textfield',
      '#title' => t('Last Name'),
    );
  }

  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Click Me'),
  );

  return $form;
}

/</strong>
 * Selects the piece of the form we want to use as replacement text and returns
 * it as a form (renderable array).
 *
 * @return renderable array (the textfields element)
 */
function ajax_example_autotextfields_callback($form, $form_state) {
  return $form['textfields'];
}

/<strong>
 * implementing hoo_validate for the form
 */
function ajax_example_autotextfields_validate($form, &$form_state) {
  //$name = $form_state['values']['textfields']['first_name'];
  if ($form_state['values']['textfields']['first_name'] == 'King Kong') {
    form_set_error('first_name', t('Ton prenom n''est pas valide'));
  }
}

/</strong>
 * hook_submit
 */
function ajax_example_autotextfields_submit($form, &$form_state) {
  $name = $form_state['values']['textfields']['first_name'];
  drupal_set_message(t('Tu as un bon prenom, %name', array('%name' => $name)));
}
?>