The ahah seems to work, but loads nothing

I'm trying to get ahah functionality like in the poll module. Except there's five fields instead of one for each request. Since I'm new to ahah I'm only trying to load one new field to begin with.
So this is my code:

$form['packages_wrapper'] = array(
      '#type'   => 'fieldset',
    '#title'  => t('Packages'),
    '#prefix' => '<div id="packages-wrapper">',
    '#suffix' => '</div>',
	 '#tree'   => TRUE, // Don't forget to set #tree!
  );
    $form['more_packages'] = array(
    '#type' => 'submit',
    '#value' => t('More packages'),
    '#description' => t("If the amount of packages above isn't enough, click here to add more packages."),
    '#weight' => 1,
    '#submit' => array('uc_posten_add_package'), // If no javascript action.
    '#ahah' => array(
	  'event' => 'click',
      'path' => ahah_helper_path(array('packages_wrapper')),
      'wrapper' => 'packages-wrapper',
      'method' => 'replace',
      'effect' => 'fade',
    ),
  );

and this is the function

function uc_posten_add_package($form, &$form_state) {
  if ($form_state['values']['more_packages']) {
	
    $form['wrapper']['length'] = array(
    '#type' => 'textfield',
    '#title' => t('Length'),
    '#size' => 10,
    '#default_value' => $form_state['wrapper']['length']
    
     );
  
  };
}

I noticed that if I remove the fieldset type from the wrapper element then nothing at all happens. As it is now, it just blinks but no field is shown.
What am I doing wrong?