Page.IsValid Not Working

I am using a regular expression validator for a textbox where
numerics should only be entered.
The error message appears when non numerics are entered, as it
should. After clicking the submit
button, I check the page.isvalid property, which is true, but should
be false, as the control is not valid.
Any ideas? Possible issues: It is any AJAX page? It is a
Thanks for any help...........Dean