Forms with date_select: How to set/leave a date to NULL?

Hi,

minor question:

If I create a form where some entries are of type date (i.e. use the form.date_select to create the form field), it does not allow to enter a NULL date. Even if the user does not enter anything, it forces a NULL date to become today's date.

Is there any (simple) way to give a date a NULL choice?

regards Hadmut