索引超出范围。必须为非负值并小于集合大小。

List<string> list1 = new List<string>();//定义一个list行的队列
        for (int i = 0; i < GridView1.Rows.Count; i++)
        {
            CheckBox cb1 = (CheckBox)GridView1.Rows[i].Cells[11].FindControl("CB1");//将GRIDVIEW中的每行的按钮的ID参数赋给 cb
            if (cb1.Checked == true)
            {
                list1.Add(GridView1.DataKeys[i].Value.ToString());//将选中的行记录到LIST中
            }
        }

        if (list1.Count <= 0)//没有选中的按钮
        {
            Response.Write("<script language='javascript'>alert('请选择要删除的项！')</script>");
            return;
        }


        string usernos = String.Join(",", list1.ToArray());
        try
        {
            using (SqlConnection conn = new SqlConnection(ConfigurationManager.AppSettings["ConStr"]))
            {
                conn.Open();
                string DeleteCmd = string.Format("delete from suppliers where supplyersid in ({0})", usernos);
                using (SqlCommand MyCommand = new SqlCommand(DeleteCmd, conn))
                {
                    MyCommand.ExecuteNonQuery();
                }
                conn.Close();
            }
            GridView1.DataBind();
            Response.Write("<script language='javascript'>alert('成功删除！')</script>");
        }
        catch
        {
            Response.Write("<script>alert('删除失败！');history.back();</Script>");
        }

高手帮帮给看看这段代码运行了之后在删除的时候，
系统报错说：
         list1.Add(GridView1.DataKeys[i].Value.ToString());
      索引超出范围。必须为非负值并小于集合大小。