MESSAGE NOT FOUND error when testing custom service in Handler Tester

We are having some weird issues when testing a custom synchronous service operation in the Handler tester.

We get the error "==>MESSAGE NOT FOUND=> (0,0)" when we hit Execute Event, and the Handler Tester returns a copy of the request message.

From using MessageBox to track what code is executing, we can see that the Handler PeopleCode object is being created (we can see MessageBox displays from inside the constructor method), but then nothing comes from our OnRequest method.

It looks almost like there is some bug in PTools that is happening after the creation of the Handler object (as seen in the execution of the constructor method), but before it calls/executes the OnRequest method.