ASP.NET : CustomValidator control does not validate!

In my asp.net page i have a textbox and two checkboxes.I added a custom validator control and set its onservervalidate property to a vb.net server side function. This function basically checks that if either one of the two checkboxes is checked, the user must enter something in the textbox. Now during runtime, if the user does not type anything in the textbox and checks the checkbox, the validation function isnt fired.On the other hand if the user types something in the textbox, the function is fired and of course the validation passes the test.

Any ideas anyone?

thanks