CCK field value deleted during checkout

Hi

I have a Computed CCK field in my content type. Before checkout, this is working fine. During checkout, the content of this field is being deleted from the database.

Code for my CCK field is:

$nid = $_GET[product_nid];
$title = db_result(db_query("SELECT title FROM {node} WHERE nid = %d", $nid));
$node_field[0]['value'] = $title;

Thanks

Glenn