Error 500 occured. /ahah_helper/<form>

An error 500 occures when you want to rebuild the whole form.

The error message returned by javascript is:

An HTTP error 500 occured.
/ahah_helper/%3Cform%3E

The code to generate this error is:

<?php
/**
 * Form builder; multistep form.
 *  
 * The form is built differently depending on which step we're on.
 * 
 * @param $form_state Standard Drupal form_state array.
 */
function ahah_multistep(&$form_state) {
  $form = array('#tree' => TRUE);
  ahah_helper_register($form, $form_state);
  
  // Display page 2 if $form_state['storage']['page_two'] is set
  if (isset($form_state['storage']['page_two'])) {
    return ahah_multistep_page_two($form);
  }
  
  // $categories = ...
  // $options = ...
  // Build stuff
  foreach ($categories as $key => $title) {
    $form['vote'][$key] = array(
      '#type' => 'slider',
      '#title' => $title,
      '#name' => $key, // TODO: define it with the $key variable.
      '#options' => $options,
    );
  }
  
  $form['next'] = array(
    '#type' => 'submit',
    '#ahah' => array(
      'event' => 'click',
      'path' => ahah_helper_path(),
      'wrapper' => 'ahah_multistep', // form id.
      'effect' => 'fade',
    ),
    '#value' => t('Next'),
  );
  
  return $form;
}
?>

When I am manually setting a path for the #ahah property the form get validated and I get the second step as it was supposed to work.

The next button becomes:

<?php
$form['next'] = array(
    '#type' => 'submit',
    '#ahah' => array(
      'event' => 'click',
      'path' => 'ahah_multistep_js',
      'wrapper' => 'ahah_multistep',
      'effect' => 'fade',
    ),
    '#value' => t('Next'),
  );
?>

In the ahah_multistep.module I add then:

<?php
/**
 * Implementation of hook_menu().
 */
function ahah_multistep_menu() {
  $items = array();

  // AJAX AHAH Multistep form.
  $items['ahah_multistep_js'] = array(
    'page callback'         => 'ahah_helper_render',
    'access callback'       => 'user_is_logged_in',
    'type'                  => MENU_CALLBACK,
  );

  return $items;
}
?>

Then it works, but I have to write this callback.