not use to ec_available

Hi.
I cannot use Availability feature.　
I think that there is this problem in the ec_availability_product_form_alter function of ec_available.module.

function ec_availability_product_form_alter(&$pform, &$node_form) {
  if (ec_product_feature_exists($node_form['#node']->ptype, 'availability')) {
    $node =& $node_form['#node'];
    $form['inventory'] = array(
      '#type' => 'fieldset',
      '#title' => t('Availability control'),
    );
    $form['inventory']['available'] = array(
      '#type' => 'textfield',
      '#title' => t('Items available'),
      '#default_value' => (($node->available != '') ? $node->available : 0),
      '#size' => 25,
      '#maxlength' => 50,
      '#description' => t('Amount of the product is which available for sale.'),
    );
    $form['inventory']['reserved'] = array(
      '#type' => 'textfield',
      '#title' => t('Items reserved'),
      '#default_value' => (($node->reserved != '') ? $node->reserved : 0),
      '#size' => 25,
      '#maxlength' => 50,
      '#description' => t('Amount of the product which has been sold, but has not been shipped or the order has not been completed'),
    );
  }
}

This problem is recovered if I convert '$form' into '$pform'.

function ec_availability_product_form_alter(&$pform, &$node_form) {
  if (ec_product_feature_exists($node_form['#node']->ptype, 'availability')) {
    $node =& $node_form['#node'];
    $pform['inventory'] = array(
      '#type' => 'fieldset',
      '#title' => t('Availability control'),
    );
    $pform['inventory']['available'] = array(
      '#type' => 'textfield',
      '#title' => t('Items available'),
      '#default_value' => (($node->available != '') ? $node->available : 0),
      '#size' => 25,
      '#maxlength' => 50,
      '#description' => t('Amount of the product is which available for sale.'),
    );
    $pform['inventory']['reserved'] = array(
      '#type' => 'textfield',
      '#title' => t('Items reserved'),
      '#default_value' => (($node->reserved != '') ? $node->reserved : 0),
      '#size' => 25,
      '#maxlength' => 50,
      '#description' => t('Amount of the product which has been sold, but has not been shipped or the order has not been completed'),
    );
  }
}

I'm developing new product module. It need ec_available.module. I expect Availability feature.