ทำ AUTO ID แล้วติด ERROR System.FormatException อ่ะครับ

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.SqlClient;


namespace test01
{
    public partial class dataCus : Form
    {
        public dataCus()
        {
            InitializeComponent();
        }

        string taxrequire;
        private void dataCus_Load(object sender, EventArgs e)
        {
            // TODO: This line of code loads data into the 'mejiDataSet.customer_status' table. You can move, or remove it, as needed.
            this.customer_statusTableAdapter.Fill(this.mejiDataSet.customer_status);
            // TODO: This line of code loads data into the 'mejiDataSet.customer_group' table. You can move, or remove it, as needed.
            this.customer_groupTableAdapter.Fill(this.mejiDataSet.customer_group);
            
            SqlConnection myConnection = default(SqlConnection);
            connection NewConnection = new connection();
            NewConnection.Connection_Today();
            connection.myConnection.Open();

            DataSet ds = new DataSet();
            SqlDataAdapter da1 = new SqlDataAdapter("SELECT * FROM customer_group", connection.myConnection);
            da1.Fill(ds, "combolist");
            txtcustgroupcode.DataSource = ds;//กำหนด DataSet
            txtcustgroupcode.DisplayMember = "combolist.ecustgroupname";
            txtcustgroupcode.ValueMember = "combolist.custgroupcode";

            DataSet ds2 = new DataSet();
            SqlDataAdapter da2 = new SqlDataAdapter("SELECT * FROM customer_status", connection.myConnection);
            da2.Fill(ds2, "combolist");
            txtcuststatuscode.DataSource = ds2;//กำหนด DataSet
            txtcuststatuscode.DisplayMember = "combolist.ecuststatusname";
            txtcuststatuscode.ValueMember = "combolist.custstatuscode";

            connection.myConnection.Close();
        }

        public string newid()
        {
            string AtCnt;
            SqlConnection myConnection = default(SqlConnection);
            connection NewConnection = new connection();
            NewConnection.Connection_Today();
            connection.myConnection.Open();

            SqlCommand myCommand = connection.myConnection.CreateCommand();
            myCommand.CommandText = "SELECT MAX(substring(custcode,2,9)) FROM customer WHERE custgroupcode = '" + txtcustgroupcode.SelectedValue.ToString() + "' ";
            myCommand.CommandType = CommandType.Text;
            AtCnt = Convert.ToString(myCommand.ExecuteScalar());
            connection.myConnection.Close();
            return AtCnt;
        }

        private void txttaxrequired1_CheckedChanged(object sender, EventArgs e)
        {
            taxrequire = "y";
        }

        private void txttaxrequired2_CheckedChanged(object sender, EventArgs e)
        {
            taxrequire = "n";
        }


        private void bntsubmit_Click(object sender, EventArgs e)
        {
            string bow = "aphinya";

            // connect database from class connection
            SqlConnection myConnection = default(SqlConnection);
            connection NewConnection = new connection();
            NewConnection.Connection_Today();
            connection.myConnection.Open();

            // command for select/insert/update/dalete
            SqlCommand myCommand = connection.myConnection.CreateCommand();
            myCommand.CommandText = "INSERT INTO customer (custcode, custgroupcode, startdate, taxid, taxrequired, custstatuscode, remark, createdate, createby) VALUES  " + " ('" + this.txtcustcode.Text + "','" + this.txtcustgroupcode.SelectedValue + "','" + this.txtstartdate.Value + "','" + this.txttaxid.Text + "','" + taxrequire + "','" + this.txtcuststatuscode.SelectedValue + "','" + bow + "','" + DateTime.Now + "','" + bow + "') ";
            myCommand.CommandType = CommandType.Text;
            myCommand.ExecuteNonQuery(); 
            connection.myConnection.Close();
            // close connect database

            MessageBox.Show("Save Successfully");

            this.Hide();
            Customer f = new Customer();
            f.Show();               
        }

        private void txtcustgroupcode_SelectedIndexChanged(object sender, EventArgs e)
        {
            txtcustcode.Text = "";
            int NewId = 1;
            string strnewid = " ";
            object obj = newid(); //DataSet Invoke from DBConnection class
            if (obj.ToString() != "")
            {
                NewId = Convert.ToInt32(obj.ToString()) + 1;
                strnewid = NewId.ToString();
                strnewid = strnewid.PadLeft(8, '0');
                txtcustcode.Text = " '"+ txtcustgroupcode.SelectedValue.ToString() + "' " + strnewid;
            }
            else
            {
                txtcustcode.Text = " '" + txtcustgroupcode.SelectedValue.ToString() + "' " + " 00000001" ;
            }

            //txtcustcode.Text = txtcustgroupcode.SelectedValue.ToString();   
            // get values from combobox[txtcustgroup] to textbox[txtcustcode]
        }


    }

}