DBCheckBox NULL value for "ValueUnchecked"....

How can i specify NULL value to ValueUnChecked property of DBCheckBox.
I want specify NULL value along with other value (say 'F'). Now whenver the
field value is NULL it looks kind of grayed.
I can assign any other multiple values to this property by delimitting the
values by ; (semicolon). But the problem here is including NULL also.

RathnaRaj