无法在asp.net MVC中为dropdownlist使用强类型参数

我是这样创建下拉列表的:

<%= Html.DropDownList(dropDownName, Model.StateProvinceSelectList, qa.AnswerValue)%>

如果有任何帮助,我们将不胜感激。

这是我的控制器:

public ActionResult Proceed(List<QuestionAnswer> questionAnswers)
{
    questionAnswer[x].AnswerValue      // <--- This is blank on postback, but if user selects, it is set properly
    ...
}

这里是html,一旦设置了值,它看起来像是MVC引擎设置了显示,而不是值(在本例中是CO):

  <select id="questionAnswers_5__AnswerValue" name="questionAnswers[5].AnswerValue">
    <option value="">CO</option>
    <option value="AL">ALABAMA</option>
    <option value="AK">ALASKA</option>
    <option value="AS">AMERICAN SAMOA</option>
    <option value="AZ">ARIZONA </option>
     ...
  </select>