Asp.NET Core Integration tests: POST method gets null values

Question

I'm trying to test my .NET Core REST api using WebClientFactory method indicated in this link. When I test the GET method everything goes well, but I don't know why when I send data through a PostAsync method it arrives as null.

Here is the Test code:

public class TravelAgentEndToEndTest : IClassFixture<WebApplicationFactory<Startup>>
{
    private readonly WebApplicationFactory<Startup> factory;
    private readonly HttpClient httpClient;

    public TravelAgentEndToEndTest(WebApplicationFactory<Startup> factory)
    {
        this.factory = factory;
        this.httpClient = factory.CreateClient();
    }



    [Fact]
    public async Task TravelAgentReceivesInvalidClientDataAndRejectsIt()
    {   
        string emptyClientData = "Bad";
        string url = "/api/clients";
        HttpResponseMessage response = await httpClient.PostAsync(url,
            new StringContent(emptyClientData,Encoding.UTF8, "application/json"));


        Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
    }
}

the POST method in my controller:

[HttpPost]
public void Post([FromBody]string value)
{
    Console.WriteLine(value);
}

When I debug the test I see that value is null. Any idea where am I making a mistake?

PS1: I tried to test the post method using Postman and it works.

PS2: The Startup class is as Visual studio created it using Web Api template under Asp.NETCore web project