Required field validator message,when the control to vlidate is still a hidden.

Hi,
     I have a required field validator, linked to a hidden text box.The page is submitting to the same page.
     It works fine for the first time.Since the text box is hidden-I do npt get the validation error message.
     But when I submit the page, the validation error message is showing up, enen if the control to vlidate is still a hidden text box?
How can I fix it so it wont show the validation error message, if the control to vlidate is still a hidden.
Thanks
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