Argh! I've gone brain dead on a Friday afternoon. To close to the weekend. I forgot the check if the nthOccurence doesn't exist:

Public Function PositionOfNthOccurence (nthOccurence as Integer, SearchString as String, SearchSubString as String) as Integer
Dim i as Integer
Dim CurrentPos as Integer
   
   CurrentPos = 0
   For i = 1 to nthOccurence
      PositionOfNthOccurence = Instr(CurrentPos + 1, SearchString, SearchSubString)
      If PositionOfNthOccurence <> 0 Then
         CurrentPos = PositionOfNthOccurence
      Else
         PositionOfNthOccurence = 0
         Exit Function
      End If
   Next 'i

End Function