.net后台下拉框绑定有一个无效SelectedValue，因为它不在项目列表中。是什么问题

 private void bangdingddl()

    {

        ddllb.DataSource = Tb_ClassMgr.hzhQueryAllTb_Classs();

        ddllb.DataTextField = "ClassName";

        ddllb.DataValueField = "ClassId";

        ddllb.DataBind();

        ddllb.Items.Insert(0, new ListItem("--请选择--", "0"));

    }

    private void bangding()

    {

        int id = Convert.ToInt32(Request.QueryString["id"]);

        ETb_GoodsInfo ea = Tb_GoodsInfoMgr.hzhQueryOneTb_GoodsInfoByGoodsId(Convert.ToString(id));

        ddllb.SelectedValue = ea.GoodsId.ToString();

    }

把下面的 ddllb.SelectedValue = ea.GoodsId.ToString();注释掉，下拉框又出来了