Fehler in Makro?

Dim wb1 As Excel.Workbook
Set wb1 = ThisWorkbook
Dim wb2 As Excel.Workbook
Set wb2 = Workbooks.Open("Z:\test.xls")
letzteZeileZiel = wb1.Sheets(2).Cells(Rows.Count, 1).End(xlUp).Row
letzteZeileQuelle = wb2.Sheets(1).Cells(Rows.Count, 6).End(xlUp).Row
For x = letzteZeileZiel To 2 Step -1
For y = letzteZeileQuelle To 2 Step -1
If wb1.Sheets(2).Range("A" & x).Value = wb2.Sheets(1).Range("G" & y).Value Then
wb1.Sheets(2).Range("A" & x).Interior.ColorIndex = 4
End If
Next
Next