Consuming Asp.Net Web API from Asp.Net Core

I have a Web API project and I am trying to call some of the API(s) from an ASP.net Core application. The code is like below:

var connectionViewModel = new ConnectionViewModel()
        {
            ConnectionId = connectionId,
            MemberId = Guid.Parse(memberId),
            ReconnectOn = DateTime.Now
        };

 HttpClient client = new HttpClient();

        client.BaseAddress = new Uri("http://localhost:63892/");

        client.DefaultRequestHeaders.Accept.Clear();

        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

        HttpResponseMessage response = client.PostAsJsonAsync("api/app/chat/connection/PostConnection", connectionViewModel).Result;

        response.EnsureSuccessStatusCode();

and here is the web api

[HttpPost]
    public async Task<IHttpActionResult> PostConnection(ConnectionViewModel viewModel)
    {
        var validation = await connectionService.CreateConnection(viewModel);

        if (validation.IsValid)
        {
            return Ok(resultSet.CreateResultSet(validation));
        }
        else
        {
            return BadRequest(validation.Message);
        }
    }

the problem is view Model is always null. I use the same code to call the web api from console application or another web api application and it works fine. But it does not work when I use inside asp.net core application.

Thanks