Create node type with asset

Hi there,

I want to create a node type with an asset field included.
I implemented hook_form:

function mymodule_form(&$node, $form_values) {

[...]

  $form['myelement'] = array(
    '#type' => 'asset',
    '#title' => t('Title'),
    '#required' => FALSE,
    '#multiple' => TRUE,
  );
}

When I now want to create a node of the new node type, I receive a Drupal-Error-Message:

warning: Missing argument 2 for theme_asset() in /[..]/sites/all/modules/asset/asset.module on line 1021.

Any ideas?

Thanks,
Stefan