问MVC视图在下拉列表中不选择空值

EN

        <div class="form-group">
        @Html.LabelFor(model => model.CarrierID, "Carrier", htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.DropDownListFor(model => model.CarrierID, (SelectList)ViewBag.CarrierList, htmlAttributes: new { @class = "form-control" })
            @Html.ValidationMessageFor(model => model.CarrierID, "", new { @class = "text-danger" })
        </div>
    </div>

public ActionResult Edit(int? id)
    {
        if (id == null)
        {
            return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
        }
        Model model= ModelService.Get(id);
        if (model== null)
        {
            return HttpNotFound();
        }
        ViewBag.CarrierList = new SelectList(db.User, "ID", "Name", model.CarrierID);

        return View(model);
    }