How can I implement a custom QueryStringConverter for RESTful WCF?

Question

I've implemented a customized QueryStringConverter class and hooked it up using a customized WebHttpBehavior subclass. When I make a service call, it hits my breakpoint in the CanConvert override (and I return true for this parameter), but it never calls my ConvertStringToValue override, and ends up just passing null to the service call... why is ConvertStringToValue never called and how can I fix it?