Strange error

def addPlacement(ships,hitListLength,start = None, mainList = None,countingList = None):
    if start == None:
    start=0
    if mainList == None:
    mainList = []
    if countingList == None:
    countingList = []
    #pop ship from array for use on this recusion level
    ship = ships.pop()
    #loop through each hit
    for x in range(start,hitListLength):
    #add this rotation to the counting list
        countingList.append(x)
    #if we don't need to go any deeper add the counting array as an element of the main list
    if len(ships) == 0:
        mainList.append(countingList)
            print "MainList: " + str(mainList)
    else:
        #otherwise recure deeper updating mainlist
        mainList += addPlacement(ships,hitListLength,(start+1),mainList,countinglist)
    #remove this loops countingList contribution so next loop can take its place
    countingList.pop()
    #return the mainlist
    ships.append(ship)
    print "MainList: " + str(mainList)
    return mainList