VB.net Using text files to create incremental numbers not working

Hi
I am testingthe function below that I am trying to create incremental invoice numbers with, ie 1,2,3,4 etc but for some reason the results are 1, 49, 54, 54, 54 etc

Sub Test
        MsgBox(oGet_Invoice_Number)
End Sub

    Function oGet_Invoice_Number() As Long
        Try
            Dim oLastInvoiceNumber As Long
            Dim oInvoiceNumber As Long
            If System.IO.Directory.Exists("c:\Program Files\KISS_Accounting") = False Then
                System.IO.Directory.CreateDirectory("c:\Program Files\KISS_Accounting")
            End If
            Dim FILE_NAME As String = "c:\Program Files\KISS_Accounting\Invoice_Number.txt"
            'So first see if text file is there - if not write the initial file with 1 as number
            If System.IO.File.Exists(FILE_NAME) = False Then
                Dim oWriter As New System.IO.StreamWriter(FILE_NAME)
                oWriter.Write("1")
                oWriter.Close()
                oGet_Invoice_Number = 1
                Exit Function
            Else
                Dim oReader As New System.IO.StreamReader(FILE_NAME)
                Dim oReadLine As String = oReader.Read()
                oLastInvoiceNumber = CLng(oReadLine)
                oReader.Close()
                oInvoiceNumber = oLastInvoiceNumber + 1
                Dim oWriter As New System.IO.StreamWriter(FILE_NAME)
                oWriter.Write(CStr(oInvoiceNumber))
                oWriter.Close()
                oGet_Invoice_Number = oInvoiceNumber
            End If
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try

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