Getting null value instead of Id when using custom AuthorizeAttribute

I call this action method using an ajax call:

[AuthorizeCheckCreator]
[HttpPost]
[OutputCache(Location = OutputCacheLocation.None, NoStore = true)]
public ActionResult Delete(Guid id)
{
    // code....
    return Content("ok");
}

I have created a custom AuthorizeAttribute for checking permission of the user (I wanted to make sure whether the user is owner the record or not):

public class AuthorizeCheckCreatorAttribute : AuthorizeAttribute
{
    public IRequest RequestService { get; set; }
    protected override bool AuthorizeCore(HttpContextBase httpContext)
    {
        RequestCheckUserViewModel request;
        if (httpContext.Request.IsAjaxRequest())
        {
            var ajaxId = JsonConvert.DeserializeObject<GetId>(System.Text.Encoding.UTF8
                                    .GetString(httpContext.Request.BinaryRead(httpContext.Request.ContentLength)));
            var currentId = ajaxId.Id;
            var isAuthorized = base.AuthorizeCore(httpContext);
            if (!isAuthorized) return false;
            requestBank = RequestService.GetUserId(Guid.Parse(currentId));
        }
        else
        {
            var rd = httpContext.Request.RequestContext.RouteData;
            var currentId = rd.GetRequiredString("id");
            var isAuthorized = base.AuthorizeCore(httpContext);
            if (!isAuthorized) return false;
            requestBank = RequestService.GetUserId(Guid.Parse(currentId));
        }

        var result = httpContext.User.Identity.GetUserId<int>() == request.UserId ||
                     httpContext.User.IsInRole("Admin") ||
                     httpContext.User.IsInRole("BankResponsible") ||
                     httpContext.User.IsInRole("BankManager");
        return result;
    }

    protected override void HandleUnauthorizedRequest(AuthorizationContext filterContext)
    {
        // handling the unauthorized requests
    }
}

public class GetId
{
    public string Id { get; set; }
}

With this code in place I always get null value for id parameter. I'm sure the client side code works fine becuase when I remove [AuthorizeCheckCreator] from Delete action method, I get value of id.
Any idea?