ASP.NET Web API controller can't handle dynamic objects?

Question

In my code I have a base class Foo and all my objects inherits from the Foo object. So let's say I have a class like this

public class Bar : Foo {
    public string Heading { get;set; }
}

I have tried to use the ApiControllers put method with dynamic but I get this error http://paste2.org/p/1914054

This is the code I'm using in the ApiController

public void Put(string id, dynamic model) {
    //do stuff
}

If I use a normal controller I can use dynamic to post data. Is it possible to add make the api controller work with dynamic or do I need to build my own model binder?

It sees like some thinks that even in MVC 3 the input parameters can't be a dynamic but that is not true and that's why I ask this question. This controller in MVC 3 works just great with dynamic as input parameter.