ASP.NET 2.0 clientvalidationfunction Firing When it Shouldn't

[mwieder]

I've got an ASP.NET form with a textbox and a customvalidator. The
form inherits from a base wizard class with back, next and cancel
buttons. The CausesValidation property of the Cancel button gets set
to false, yet the javascript clientvalidationfunction is firing when
the cancel button is hit. Running through the debugger, the
CausesValidation property is set to false on the Page_Load of the
derived form. Why is the clientside javascript firing even though
causesvalidation is false? How do I stop this?