HttpRequest object is null in response from Azure Function

Question

This is my Azure function:

    [FunctionName("CreateItem")]
    public static async Task<IActionResult> Run(
        [HttpTrigger(AuthorizationLevel.Anonymous, "post", Route = null)] HttpRequest req, ILogger log)
    {
        string CompanyId = req.Query["CompanyId"];
        string Table = req.Query["Table"];
        string RowId = req.Query["RowId"];
        string Key = req.Query["Key"];
        string Action = req.Query["Action"];
        string Arg = req.Query["Arg"];

        string requestBody = await new StreamReader(req.Body).ReadToEndAsync();
        dynamic data = JsonConvert.DeserializeObject(requestBody);

When I look at the requestBody objcet in runtime, it is null. Why Is that?