System.ArgumentOutOfRangeException: StartIndex 不能小于 0。

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            string str = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Server.MapPath("../../App_Data/tuohaichuanmei.mdb") + ";Persist Security Info=False";
            OleDbConnection conn = new OleDbConnection(str);
            conn.Open();
            OleDbCommand cmd = new OleDbCommand("select * from News where ID="+Request["id"].ToString()+"",conn);
            OleDbDataReader rad = cmd.ExecuteReader();           
            if (rad.Read())
            {
                BiaoTi.Text = rad["Title"].ToString();
                NeiRong.Text = rad["Body"].ToString();
                HiddenField1.Value = rad["Img"].ToString();
            }
            rad.Close();
            conn.Close();
        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string str = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Server.MapPath("../../App_Data/tuohaichuanmei.mdb") + ";Persist Security Info=False";
        OleDbConnection conn = new OleDbConnection(str);
        conn.Open();
        string Img = string.Empty;
        if (FileUpload1.HasFile == true)
        {
            string tif = Img.Substring(Img.LastIndexOf('.'));
            string NewName = DateTime.Now.ToString();
            string NewStr = NewName.Replace('-', '2').Replace(':', '2').Replace(' ', '2');
            FileUpload1.SaveAs(Server.MapPath("~/Admin/Img/" + NewStr + tif));
            File.Delete(Server.MapPath("~/Admin/Img/" + HiddenField1.Value));
            OleDbCommand cmd = new OleDbCommand("update News set Title='"+BiaoTi.Text+"',Body='"+NeiRong.Text+"',Img='"+NewStr+tif+"' where ID="+Request["id"].ToString()+"", conn);
            cmd.ExecuteNonQuery();
        }
        else
        {
            OleDbCommand cmd = new OleDbCommand("insert into News(Title,Body,Img)values('" + BiaoTi.Text + "','" + NeiRong.Text + "')", conn);
            cmd.ExecuteNonQuery();
        }
        conn.Close();
    }
请教高人指点 事成之后必有重谢