Html 没有类型为'；IEnumerable<；选择列表项>'；那有钥匙'；CompPlanTypeId'；

我不明白为什么会出差错。 如果我删除保存Comp Plan:的div标记，它就可以正常工作

<div id="tabs-1"> 
    <form id="importForm">
        <div id="required">
            <div>
                <label class="OptionLabel">Comp Plan:</label>
                @*// TODO: suspect this next line is causing problems*@ 
                @Html.DropDownListFor(model => model.CompPlanTypeId, (IEnumerable<SelectListItem>)ViewBag.CompPlanList)
            </div>
            <div>
                <label class="OptionLabel">System:</label>
                @Html.DropDownListFor(model => model.SystemId, (IEnumerable<SelectListItem>)ViewBag.SystemList)
            </div>
            <div>
                <label class="OptionLabel">Month:</label>
                @Html.DropDownListFor(model => model.Month, (SelectList)ViewBag.MonthList)
            </div>
            <div>
                <label class="OptionLabel">Year:</label>
                @Html.DropDownListFor(model => model.Year, (SelectList)ViewBag.YearList)
            </div>
        </div>

ViewBag.CompPlanList设置为2项。一个包含空白字符串，一个包含下拉值

这是堆栈跟踪

[InvalidOperationException: There is no ViewData item of type 'IEnumerable<SelectListItem>' that has the key 'CompPlanTypeId'.]

[InvalidOperationException:没有具有键“CompPlantTypeId”的“IEnumerable”类型的ViewData项。]

System.Web.Mvc.Html.SelectExtensions.GetSelectData（HtmlHelper HtmlHelper，字符串名）+392896 System.Web.Mvc.Html.SelectExtensions.SelectInternal（HtmlHelper HtmlHelper、字符串选项标签、字符串名称、IEnumerable

1选择列表、布尔allowMultiple、IDictionary

2 htmlAttributes）+137 System.Web.Mvc.Html.SelectExtensions.DropDownListFor（HtmlHelper

1 HtmlHelper，Expression

1 Expression，IEnumerable

1选择列表，字符串选项标签，IDictionary

2 htmlAttributes）+35 System.Web.Mvc.Html.SelectExtensions.DropDownListFor（HtmlHelper

1 HtmlHelper，Expression

1 Expression，IEnumerable`1 selectList）+60 c:\WIP\CompensationAudit\src\WebLibrary\Views\Import\Index.cshtml:28中的ASP.\页面\视图\导入\索引\ cshtml.Execute（）

[InvalidOperationException: There is no ViewData item of type 'IEnumerable<SelectListItem>' that has the key 'CompPlanTypeId'.]