ASP.NET Custom Validator Client side & Server Side validation not firing!

Your CustomValidator will only fire when the TextBox isn’t empty.

If the input control is empty, no validation functions are called and validation succeeds. Use a RequiredFieldValidator control to require the user to enter data in the input control.

A thread from Stackoverflow:

http://stackoverflow.com/questions/701029/asp-net-custom-validator-client-side-server-side-validation-not-firing