Frustrating: GWT InvocationException

I am having some success with GWT - but here's one that took me a couple hours to figure out. I created an asynchronous RPC service. Pretty much straight from the "book". Wouldn't work - wouldn't work. Tore everything back to the bare bones. (OK - the service was taking a user-defined class as an argument and returning a list.) I changed it to take a String and return a String. Still always got the InvocationException - with no cause. Finally also ripped the RemoteServiceServlet back to the basics -- did I mention I implemented the init and service methods to allow setting up other stuff I needed? Lo and behold it started working! Doh!
In the simple minded version I originally wrote it wasn't going to actually service anything, just initialize. By calling super.service(request, response) things went much better. Sometimes it IS just the simple problems.