MVC4型号状态无效

我在mvc4+EF6项目中发布时遇到问题。视图中的模型填充了除年份和期间之外的所有字段(它们是下拉列表字段(。在附加到视图的视图模型中，字段Year和Period未设置为DataAnnotations所必需。但是，当我尝试发布时，ModelState是无效的。

如何确定ModelState无效的原因？

参见以下代码：

HTML:

@using (Html.BeginForm())
            {
                @Html.AntiForgeryToken()
                @Html.HiddenFor(model => model.Id)
                @Html.HiddenFor(model => model.Status)
                <div class="form-horizontal">
                    <fieldset>
                        <legend>Edit Performance Cycle Details</legend>
                        <div class="form-group">
                            @Html.LabelFor(model => model.Year, new { @class = "col-lg-2 control-label" })
                            <div class="col-lg-10">
                                @Html.DropDownList("Year", new SelectList(ViewBag.Years), "--- Select Cycle Year ---", new { @class = "form-control", @id = "Year" })
                            </div>
                        </div>
                        <div class="form-group">
                            @Html.LabelFor(model => model.Period, new { @class = "col-lg-2 control-label" })
                            <div class="col-lg-10">
                                @Html.DropDownList("Period", new SelectList(ViewBag.Period, "Id", "Title"), "--- Select Period ---", new { @class = "form-control", @id = "Period" })
                            </div>
                        </div>
                        <div class="form-group">
                            @Html.LabelFor(model => model.StartDate, new { @class = "col-lg-2 control-label" })
                            <div class="col-lg-10">
                                @Html.EditorFor(model => model.StartDate, "EditorForCycleSD")
                            </div>
                        </div>
                        <div class="form-group">
                            @Html.LabelFor(model => model.CloseDate, new { @class = "col-lg-2 control-label" })
                            <div class="col-lg-10">
                                @Html.EditorFor(model => model.CloseDate, "EditorForCycleCD")
                            </div>
                        </div>
                        <div class="form-group">
                            <div class="col-lg-10 col-lg-offset-2">
                                @Html.ActionLink("Cancel", "Index", "Cycle", null, new { @class = "btn btn-default" })
                                <button class="btn btn-primary" type="submit">Submit</button>
                            </div>
                        </div>
                    </fieldset>
                </div>
            }

控制器代码：

[HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Edit([Bind(Include = "StartDate, CloseDate, Id, Status")] EditCycle cycleUpdate, string Year, string Period)
    {
        try
        {
            if (ModelState.IsValid)
            {
                int yearPost = Int32.Parse(Year);
                string periodPost = Period.Trim();
                switch (periodPost)
                {
                    case "1":
                        periodPost = yearPost.ToString() + " - H1";
                        break;
                    case "2":
                        periodPost = yearPost.ToString() + " - H2";
                        break;
                    default:
                        return View(cycleUpdate);
                }
                int pf = Int32.Parse(User.Identity.Name.Substring(8));
                cycleUpdate.Year = yearPost;
                cycleUpdate.Period = periodPost;
                if (this.repo.EditCycle(cycleUpdate, pf))
                {
                    return View("Index");
                }
                else
                {
                    List<CyclePeriod> period = new List<CyclePeriod>{
                        new CyclePeriod {Id=1, Title="First Half : January 1 - June 30"},
                        new CyclePeriod {Id=2, Title="Second Half : July 1 - December 31"}
                    };
                    int[] years = new int[5];
                    for (int i = (DateTime.Now.Year - 2), j = 0; i <= (DateTime.Now.Year + 2); i++, j++)
                    {
                        years[j] = i;
                    }
                    ViewBag.Years = years;
                    ViewBag.Period = period;
                    return View(cycleUpdate);
                }
            }
            else
            {
                List<CyclePeriod> period = new List<CyclePeriod>{
                    new CyclePeriod {Id=1, Title="First Half : January 1 - June 30"},
                    new CyclePeriod {Id=2, Title="Second Half : July 1 - December 31"}
                };
                int[] years = new int[5];
                for (int i = (DateTime.Now.Year - 2), j = 0; i <= (DateTime.Now.Year + 2); i++, j++)
                {
                    years[j] = i;
                }
                ViewBag.Years = years;
                ViewBag.Period = period;
                return View(cycleUpdate);
            }
        }
        catch (Exception)
        {
            List<CyclePeriod> period = new List<CyclePeriod>{
                    new CyclePeriod {Id=1, Title="First Half : January 1 - June 30"},
                    new CyclePeriod {Id=2, Title="Second Half : July 1 - December 31"}
                };
            int[] years = new int[5];
            for (int i = (DateTime.Now.Year - 2), j = 0; i <= (DateTime.Now.Year + 2); i++, j++)
            {
                years[j] = i;
            }
            ViewBag.Years = years;
            ViewBag.Period = period;
            return View(cycleUpdate);
        }
    }

var errors = ModelState.Where(x => x.Value.Errors.Any())
                .Select(x => new { x.Key, x.Value.Errors });