下拉列表 - 输入字符串的格式不正确

Question

我的下面是metod：

private void BindModels(int makeID, int typeID)
{
    try
    {
        DataSet ds = (DataSet)ViewState["ds_DDLsData"];
        EnumerableRowCollection models;
        if (typeID != null)
        {
            models = from dt_Models in ds.Tables[10].AsEnumerable()
                         where dt_Models.Field<int>("MakeID") == makeID & dt_Models.Field<int>("TypeID") == typeID
                         select new
                         {
                             modelName = dt_Models.Field<string>("ModelName"),
                             modelID = dt_Models.Field<int>("ModelID")
                         };
        }
        else
        {
            models = from dt_Models in ds.Tables[10].AsEnumerable()
                     where dt_Models.Field<int>("MakeID") == makeID
                     select new
                     {
                         modelName = dt_Models.Field<string>("ModelName"),
                         modelID = dt_Models.Field<int>("ModelID")
                     };
        }

        ddlModel.DataSource = models;
        ddlModel.DataTextField = "ModelName";
        ddlModel.DataValueField = "ModelID";
        ddlModel.DataBind();
        ddlModel.Items.Insert(0, new ListItem("-- Select Model --", "-1"));
        ddlModel.SelectedIndex = 0;
    }
    catch (Exception ex)
    {
        var error = new JavaScriptSerializer().Serialize(ex.Message.ToString());
        ScriptManager.RegisterStartupScript(this, this.GetType(), "myalert", "alert('" + error + "');", true);
        methods.LogErrors(Session["user"].ToString(), this.Page.ToString(), MethodBase.GetCurrentMethod().Name.ToString(), ex.ToString());
    }
}

我打来电话：

    protected void ddlMake_SelectedIndexChanged(object sender, EventArgs e)
{
    try
    {
        if (pnlModels.Visible == false)
        {
            pnlModels.Visible = true;
        }

        BindModels(Convert.ToInt32(ddlMake.SelectedValue), Convert.ToInt32(ddlType.SelectedValue));
    }
    catch (Exception ex)
    {
        var error = new JavaScriptSerializer().Serialize(ex.Message.ToString());
        ScriptManager.RegisterStartupScript(this, this.GetType(), "myalert", "alert('" + error + "');", true);
        methods.LogErrors(Session["user"].ToString(), this.Page.ToString(), MethodBase.GetCurrentMethod().Name.ToString(), ex.ToString());
    }
}

问题是在某些情况下，类型下拉列表可以为空。我的问题是BindModels方法应该是什么样子，因为现在我收到错误 - 输入字符串的格式不正确。在此先感谢您的回复。