No MediaTypeFormatter is available

You may have come across the error message No MediaTypeFormatter is available to read an object of type T from content with media type 'text/plain'. during your adventures in ASP.NET Web API. Tonight it was my turn to be faced with this bothersome little 'issue'.

I was attempting to post {Username:"testuser", Password:"testpass"} to an API controller which received a ViewModel like so:

public class UserViewModel
{
    public string Username { get; set; }
    public string Password { get; set; }
}

public HttpResponseMessage Post(UserViewModel value)
{
    // do something
}

The only problem was that it just wasn't finding the controller action. Spitting out error messages and generally being counter-productive.

After going through lots of StackOverflow posts and dismissing them, it turned out the first one was actually relevant. I was using a Chrome extension (Postman) to help with REST calls and even though I had selected JSON, it was still sending requests as plain/text, and not application/json.

I set the Content-type header to application/json and it started working.