var kategorie_wlasna = new SelectList(
  (from z in Model.Kategoria
    where !formReadOnly || z.Id == Model.KategoriaWlasnaId 
      select z),"Id","Nazwa");
...
@Html.DropDownListFor(
  model => model.KategoriaWlasnaId,kategorie_wlasna,"----",htmlClassDropDownListDef)

在我的视图模型我有属性没有任何注释为必需：

public long KategoriaWlasnaId { get; set; }

但是该字段仍然是必需的。在浏览器中我得到：

<select class="input-validation-error form_object1" data-val="true" data-val-number="The field KategoriaWlasnaId must be a number." data-val-required="The KategoriaWlasnaId field is required." id="KategoriaWlasnaId" name="KategoriaWlasnaId">
  <option value="">-----</option>
  <option value="1227">Wykroczenie</option>
  <option value="1228">Przestępstwo</option>
</select>

我失踪了什么