RSVP reply form missing

When the invitee clicks on the second link in the email, presumably to indicate their RSVP status, the form doesn't display the "Your Reply" block. The relevant code is:

function rsvp_reply_form($hash) {
  $invite = _rsvp_get_invite($hash);
  $responses = array('yes' => t('Yes'), 'no' => t('No'), 'maybe' => t('Maybe'));
  if ($invite->response == 'none') {
    $responses['none'] = t('None');
  }

  $form = array();
  $form['invite_reply'] = array(
      '#type' => 'select',
      '#title' => t('Your Reply'),
      '#default_value' => $invite->response,
      '#options' => $responses,
      '#description' => t('Select your response to the invitation here.')
      );
  $form['hash'] = array(
      '#type' => 'hidden',
      '#value' => $hash
      );

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

  return theme('rsvp_reply', drupal_get_form('rsvp_reply_form', $form));
}

What's wrong with this?