Warning message after hiding CCK-Date field

Hi! After hiding CCK-Date field (provided by http://drupal.org/project/date) I see a warning message:

warning: array_shift() [function.array-shift]: The argument should be an array in [.....]\www\includes\form.inc on line 1326

This message is appearing on the Manage Form page and on the Node Editing page. Drupal core version: 6.13.

To see it you should do the following:
1. Install CCK & Date modules.
2. Add Date field for any node type (Date module provides 3 new field types: Datestamp, Datetime, Date. I've used Datetime).
3. Hide this field on the Manage Form page and you will see the warning.