ผมครีโค้ด autonumber แล้ว error ดังนี้ครับ รบกวนช่วยชี้แนะด้วยครับ

Sub autoid()
        Dim sqlBill As String = ""
        Dim Year As Integer = Date.Now.Year
        Try
            sqlBill = "select cost_id FROM Cost order by cost_id"
            connect()
            With da
                .SelectCommand = New OleDbCommand(sqlBill, conn)
                .Fill(ds, "cost_id")
            End With

            Dim count As Integer = ds.Tables("cost_id").Rows.Count
            If (count > 0) Then
                Dim ID As String = ds.Tables("cost_id").Rows(count - 1).Item("cost_id")
                Dim num As String = Convert.ToInt32(id.Substring(5, 10))
                num += 1
                txtcost_id.Text = num.ToString("C" & Year.ToString & "00000")
            ElseIf (count = 0) Then
                txtcost_id.Text = "C" & Year.ToString & "00001"
            End If

        Catch ex As Exception
            MessageBox.Show("เกิดข้อผิดพลาด เนื่องจาก " & ex.Message, "ผลการตรวจสอบ", MessageBoxButtons.OK, MessageBoxIcon.Information) 'โชว์ messagebox เเจ้งผู้ใช้
            conn.Close()
        End Try
    End Sub