Error: 'An illegal choice has been detected' for unchecked mandatory multiple select

- Create new webform (version 4.7 and CVS 2006-10-23 tested)
- add one component: select; multiple; mandatory; no default value; put 2 options in it

It should render as two checkboxes.

Do not set any of the checkboxes (keep them clear) and submit the form.

You'll receive: An illegal choice has been detected. Please contact the site administrator.
Expected behavior: 'XX field is required' validation error

Any help on this?
Thanks a lot,
--Josef