Server side validation handler not called for Custom Validator

[Hunpin Toh]

I have a page with a textbox. I'm using a required field
validator, as well as a server side custom validator to
validate user's input in the textbox. The required field
validator is working fine but the custom validator isn't.
The validation handler for the customer validator isn't
getting called. This is the same piece of code I've used
in many other places in the same app so I don't
understand why the custom validator isn't firing the
event. Any help is appreciated. Thanks.