Setting Content-Type header on WebClient.OpenWrite generates WebException

Question

I am using WebClient.OpenWrite to POST to a WCF HTTP Web service in .NET 4. If I set the content type on the WebClient headers, then I wind up with a WebException (400 - Bad Request) when the stream is closed.

var client = new WebClient();

// Setting ContentType generates a WebException on 400 - Bad Request when the stream is closed
//client.Headers[ HttpRequestHeader.ContentType ] = "application/json";

var stream = client.OpenWrite( "http://localhost:21159/Colors.svc/Color", "POST" );

var serializer = new DataContractJsonSerializer( typeof( ClientColor ) );
serializer.WriteObject( stream, color );
stream.Close();

The service interface looks like this:

[ServiceContract]
public interface IColors
{
  [WebInvoke( UriTemplate = "Color", Method = "POST" )]
  void ColorPost( Stream color );
}

If I set the Content-Type header to "application/xxx", then the POST works correctly.

Any ideas why I get the WebException when the Content-Type is "application/json"?