Rechnungsnummer und Probleme mit MID

Sub test()
Dim i As Integer
Dim zk As Integer
Dim strDatei As String
Dim rechnummer As String
Dim nummax As Integer
With Application.FileSearch
.NewSearch
.LookIn = "D:\projekt\"
.Filename = "*.doc"
.SearchSubFolders = False
If .Execute(SortBy:=msoSortByFileName, sortorder:=msoSortOrderAscending) > 0 Then
For i = 1 To .FoundFiles.Count
strDatei = .FoundFiles(i)
zk = InStr(1, strDatei, "100")
rechnummer = Mid(strDatei, zk, 6)
rechnummer2 = CInt(rechnummer)
If rechnummer2 > nummax Then nummax = rechnummer2
Next i
End If
End With
End Sub