Error - The submitted request did not contain a valid XML document.

I have a controller only action in webmaker (no form after controller) that I am accessing via a Agent call in bizflow. I can call the url via browser and webmaker service tester fine. But when the Bizflow agent tries to call it I am getting the error "The submitted request did not contain a valid XML document.".

So two questions would be 1) what is causing this on the webmaker side, and 2) would you know if it is possible to call a controller only via a Bizflow agent? The agent is calls a server-side url.