vba run time error 9 subscript out of range

Я вызываю метод с main_report - название файла

Sub main(main_report, semi_report, period)
Set wb = ThisWorkbook
Set wbb = Workbooks(parse_name(main_report))
...

и получаю ошибку run time error 9

сама функция parse_name

Function parse_name(wb_name)
listik = Split(wb_name, ".")
new_name = ""
For i = 0 To UBound(listik) - 1
    new_name = new_name & listik(i) & "."
Next i
new_name = Left(new_name, Len(new_name) - 1)
parse_name = new_name
End Function