Object becomes null after calling HTTPClient.PostAsJsonAsync to access Web Api - asp.net

Question

In my web form (ASP .Net 4.8, VB .Net), I have calling the following function from code-behind submit sales transaction via web api:
Public Async Function SendSale(fs As SaleTxnReqModel) As Task(Of SaleTxnRespModel)
Dim returnData As SaleTxnRespModel = New SaleTxnRespModel()
Dim apiUrl = "Sales/SendSale"
Dim resultRemark As String = String.Empty
Try
Using client = New HttpClient()
client.BaseAddress = New Uri(AppComp.NormalizeBaseUrl(constWebApiUri))
client.DefaultRequestHeaders.Accept.Clear()
client.DefaultRequestHeaders.Accept.Add(New MediaTypeWithQualityHeaderValue("application/json"))
Dim jsonString As String = JsonConvert.SerializeObject(fs)
Dim content = New StringContent(jsonString, Encoding.UTF8, "application/json")
Dim response As HttpResponseMessage = Await client.PostAsJsonAsync(apiUrl, content)
returnData = Await response.Content.ReadAsAsync(Of SaleTxnRespModel)()
If Not response.IsSuccessStatusCode Then
resultRemark = AppComp.SetWebServStatusText(response.StatusCode)
End If
End Using
Catch ex As Exception
resultRemark = AppComp.GetExceptionMessage(ex)
Finally
returnData.REMARK = resultRemark
End Try
Return returnData
End Function
At the receiving web api, the request object has become null. 
[Route("SendFuelSale")]
[HttpPost]
public IActionResult SendFuelSale(SaleTxnReqModel txn)
{
// Process sales transaction...
}
I have tried using SendAsync method and it worked:
Dim jsonString As String = JsonConvert.SerializeObject(fs)
Dim content = New StringContent(jsonString, Encoding.UTF8, "application/json")
Dim request = New HttpRequestMessage(HttpMethod.Post, apiUrl)
request.Content = content
Dim response As HttpResponseMessage = Await client.SendAsync(request).ConfigureAwait(False)
I just wonder what went wrong when using PostAsJsonAsync  method.
Any idea? Please adivse. Thanks.