Populate doctrine entity using fos rest bundle in post request

Question

I have the following action:

/**
 * @param Agent $agent
 * @Post("/agents")
 * @ParamConverter(class="DnDRaHApiBundle:Agent")
 */
public function postAction(Agent $agent)
{

  //return $request->request->all();
}

But when I hit the url with a post request and a json body, I get this error:

Unable to guess how to get a Doctrine instance from the request information.

What can I do to fix this, I've spent now a few hours in this. Documentation is really poor.

Thanks.