Fire OnServerValidate Method When No Value Exists in Textbox

[Rigs]

Hi,

I have a textbox (txt1) with a Custom Validator (cv1) on a web form.
The problem is that cv1's OnServerValidate method only fires when text
exists in the textbox that cv1 is assigned to validate, and the user
clicks the submit button. I'd like to have cv1's OnServerValidate
method fire whenever the submit button is clicked on.

Can anyone offer an explanation why this occurs and how I can have
that method fire whenever the submit button is clicked? Please help!

Thanks,
-Rigs