การรันรหัส เช่น S00001 vb.net แล้วรันได้แค่ S00009 แล้ว error แก้ยังไงคับ รบกวนทีคับ

Public Function a1() As String
        Dim id1 As String
        Dim l1 As String
        Dim dt1 As DataTable = New DataTable
        Dim SQL1 As String
        SQL1 = "SELECT Max(song_id) FROM song"
        dt1 = c.ExRead(SQL1)
        id1 = dt1(0)(0).ToString
        If dt1(0)(0).ToString <> "" Then
            If id1.Substring(id1.Length - 1) <> "9" Then
                id1 = dt1(0)(0).ToString
                l1 = id1.Substring(id1.Length - 1, id1.Length - 6)
                id1 = id1.Substring(0, id1.Length - 1) + (Integer.Parse(l1) + 1).ToString
            ElseIf dt1(0)(0).ToString.Substring(id1.Length - 1) = "9" Then
                If dt1(0)(0).ToString.Substring(id1.Length - 2, id1.Length - 6) = "9" Then
                    If dt1(0)(0).ToString.Substring(id1.Length - 3, id1.Length - 6) = "9" Then
                        If dt1(0)(0).ToString.Substring(id1.Length - 4, id1.Length - 6) = "9" Then
                            If dt1(0)(0).ToString.Substring(id1.Length - 5, id1.Length - 6) = "9" Then
                                id1 = dt1(0)(0).ToString
                                l1 = Integer.Parse(id1.Substring(1, id1.Length - 6)) + 1
                                id1 = "S" + l1 + "00000"
                            Else
                                id1 = dt1(0)(0).ToString
                                l1 = Integer.Parse(id1.Substring(2, id1.Length - 6)) + 1
                                id1 = "S0" + l1 + "0000"
                            End If
                        Else
                            id1 = dt1(0)(0).ToString
                            l1 = Integer.Parse(id1.Substring(3, id1.Length - 6)) + 1
                            id1 = "S00" + l1 + "000"
                        End If
                    Else
                        id1 = dt1(0)(0).ToString
                        l1 = Integer.Parse(id1.Substring(4, id1.Length - 6)) + 1
                        id1 = "S000" + l1 + "00"
                    End If
                Else
                    id1 = dt1(0)(0).ToString
                    l1 = Integer.Parse(id1.Substring(5, id1.Length - 6)) + 1
                    id1 = "S0000" + l1.ToString + "0"
                End If
            End If
        ElseIf dt1(0)(0).ToString = "" Then
            id1 = "S00001"
        End If
        Return id1
End Function