Trying to hide nodereference field --> "This post can't be referenced"

Hi,

In a view for showing documents I've added a form for adding an item to the currently shown document. The form contains a node reference that points to the current node:

$form['field_node_referentie']['nids']['#default_value'][0] = arg(1);

This works without problems. As the user doesn't need to select a node reference manually in this case, I'd like to hide the node reference field. So I tried several variations of this:

$form['field_node_referentie']['#type'] = 'hidden';
$form['field_node_referentie']['nids']['#value'][0] = arg(1);

A title field can be hidden this way, so why not a node reference field? But in this case an error is displayed: "This post can't be referenced."

Apparently my experiments aren't accepted...

How can I hide the node reference field?