How to use HttpClient.PutAsJsonAsync()

I have a c# project with a RESTfull client and RESTfull server. I am unable to get PUT request to work. I think I am maybe not using PutAsJsonAsync() correctly.

The client...

        using System.Net.Http;
        using System.Net.Http.Formatting;

        public class RequestParams {
             public String param1 { get; set;}
        }


        fullUrl = "http://localhost:3578/api/v1.0/config";
        client = new HttpClient();
        var requestParams = new RequestParams {
            parm1 = "value1"
        };
        response = client.PutAsJsonAsync(fullUrl, requestParams).Result;
        var result = response.Content.ReadAsStringAsync().Result;

The server (nancy self hosting)...

        Put["/api/v1.0/config"] = x =>
        {
                NameValueCollection requestParams =
                    HttpUtility.ParseQueryString(Request.Body.AsString());
                Console.WriteLine("requestParams=" + requestParams);
                var param1 = requestParams["param1"];

At the server side I get for the requestParams...

{"param1":"value1"}={"param1":"value1"}

I have confirmed that the server is working by using a python client. I get for requestParams...

param1=value1