MVC3尽管没有[Required]属性，但仍然需要“属性字段”错误

I have this code:

我有这个代码：

@this.Html.DropDownListFor(vm => vm.FishId, 
    new SelectList(this.Model.Fishies, "FishId", "FishName", this.Model.FishId), "Please Select a Fish")

@this.Html.ValidationMessageFor(vm => vm.FishId)
@this.Html.DropDo