DropDownListFor 驗證失效

mvc3 下 DropDownList驗證失效加入以下即可解決
new { data_val_required = "請輸入類別", data_val = "true" }

@Html.DropDownListFor(m => m.DepartmentID, (SelectList)ViewBag.DepartmentID, "==請選擇部門==", new { data_val_required = "*", data_val = "true" })