Ошибка 'NoneType' object is not subscriptable

            for i in range (len(transport)+1):
                print ('Transport: ' + str (transport[i]) + '   That transport cost ' + str (transport_cost[i]) + ' (if you want to buy it, you can write user ' + str (transport_users [i]) + ').')

пишу так, выдает ошибку:

'NoneType' object is not subscriptable

Спасибо!