I keep getting the following error for this activity:

Oops, try again. trip_cost('Charlotte', 4) raised an error: cannot concatenate 'str' and 'NoneType' objects

```
def hotel_cost(days):
return 140*days
def plane_ride_cost(city):
if city == "Charlotte":
return 183
elif city == "Tampa":
return 220
elif city == "Pittsburgh":
return 222
elif city == "Los Angeles":
return 475
def rental_car_cost(days):
cost = 40*days
if days >= 7:
return cost - 50
elif days >=3:
return cost - 20
return cost
def trip_cost(days, city):
return hotel_cost(days) +plane_ride_cost(city) + rental_car_cost(days)
```