Using a class method in a NonlinearIOSystem #629

I am trying to use a class method in a NonlinearIOSystem. To test, I adapted the predator-prey example such that the predprey_rhs function becomes a method in the new predprey class. Then, I create an instance of the predprey class (pp), and create the NonlinearIOSystem using the pp.predprey_rhs method. All good to that point. When I try to calculate the open-loop response with control.input_output_response, I get "TypeError: dynamics() takes 4 positional arguments but 5 were given". The method predprey_rhs has the self argument in its definition, so that shouldn't be a problem. Anyone have any ideas?