null value in date field - Mailing list pgsql-novice

I have a form that contains optional DATE fields.  To enter null
values, I've tried null, 'null', '', '-', '0', 0, and each time I get
one of the following errors:

ERROR:  Bad numeric input format ''

ERROR:  Bad date external representation ''

Is there a correct way to enter null values into DATE fields?

Thanks,
Jeff Deroshia