[VBA] Problème de variable non définie

Function get_driving_time(place_a, place_b)
my_xml_path = "http://maps.google.fr/maps?saddr=" & place_a & "&daddr=" & place_b & "&ie=utf-8&v=2.1&cv=4.0.2744&hl=fr&output=kml"
Set xmlDoc = CreateObject("MSXML2.DOMDocument")
xmlDoc.async = False
xmlDoc.Load (my_xml_path)
Set nodelist = xmlDoc.getElementsByTagName("description")
my_raw_string = nodelist.Item(nodelist.Length - 1).firstchild.nodevalue
get_driving_time = Monextract(my_raw_string, "environ ", ")")
get_driving_time = Replace(get_driving_time, "minutes", "")
If InStr(get_driving_time, "heure") = 0 Then get_driving_time = "00:" & get_driving_time
get_driving_time = Replace(get_driving_time, "heures", ":")
get_driving_time = Replace(get_driving_time, "heure", ":")
get_driving_time = get_driving_time & ":00"
get_driving_time = Replace(get_driving_time, " ", "")
get_driving_time = TimeValue(get_driving_time)
End Function

Function Monextract(machaine, debut, fin)
PosH1 = InStr(1, machaine, debut)
PosH2 = InStr(1, machaine, fin)
long_first = Len(debut)
Leng = PosH2 - PosH1 - long_first
Monextract = Mid(machaine, PosH1 + long_first, Leng)
End Function