Error 415 Unsupported Media Type error when using json-Post

ServerParams s1 = new ServerParams(serverParams);
           RestClient cl1 = new RestClient("http://1.2.3.4");
           cl1.AddHandler(contentType, new RestSharp.Deserializers.JsonDeserializer());
           RestRequest r1 = new RestRequest(RestSharp.Method.POST);
           r1.RequestFormat = DataFormat.Json;
           r1.AddHeader("X-Serenity-User", "admin");
           r1.AddHeader("X-Serenity-Password","admin");
           r1.Resource = resource;
           r1.AddHeader("Accept","application/vnd.resource+json");
           r1.AddHeader("Content-Type","application/vnd.resource+json");
           r1.AddObject(viewObject);
           IRestResponse response = cl1.Execute(r1);
           ValidateResponse(response);
           return response;