[Zope] Null value from Form Input

[Montagne, Michael]

I have a form with a series of radio buttons that allow rating 1-5.  In
order to make it easier to get an average of those numbers, I'd like to
allow NULL inputs.  A snippet of my SQL method to update the database is:
INSERT INTO tblContactEval (responsiveness)
VALUES(<dtml-sqlvar responsiveness type="int" optional>);

The form looks something like this:
<input type=radio name=responsiveness value=1>1&nbsp
<input type=radio name=responsiveness value=2>2&nbsp
<input type=radio name=responsiveness value=3>3&nbsp
<input type=radio name=responsiveness value=4>4&nbsp
<input type=radio name=responsiveness value=5>5&nbsp

When I do not check any radio button in that group I get a "Bad Request"
error citng the "responsiveness" variable.  How can I get a NULL value into
my database for those situations where this category does not apply?

thanks,
mjm