error 'Conversion from type dbnull to type integer is not valid'

Public Sub RunID()
        Dim da As SqlDataAdapter = Nothing
        Dim ds As New DataSet
        Dim dt As DataTable = Nothing
        Dim sql As String = ""
        Dim a As String = "0000000001"



        ConnString = "Data Source=xxx\MSSQLSERVER2008;Initial Catalog =BPCS2FMIS;Integrated Security=SSPI;"

        Try
            Conn.ConnectionString = ConnString
            Conn.Open()

            sql = "SELECT max(Cust_No)+1 as id FROM CustomerMaster"


            Dim Cmd As New SqlCommand(sql, Conn)
            Dim Rs As SqlDataReader = Cmd.ExecuteReader
            Dim i As Integer

            If Rs.Read Then
                i = Rs("id")
                txtCustNo.Text = i.ToString("0000000000")

            End If
        Catch ex As Exception
            MessageBox.Show(ex.Message, "ข้อมูลผิดพลาด", MessageBoxButtons.OK, MessageBoxIcon.Exclamation)
        End Try
        Conn.Close()

    End Sub