Part of the whole

Oops, try again. get_class_average([alice]) resulted in an error: 'NoneType' object is not iterable

i have no idea what is wrong, or what is Nonetype

```
lloyd = {
"name": "Lloyd",
"homework": [90.0, 97.0, 75.0, 92.0],
"quizzes": [88.0, 40.0, 94.0],
"tests": [75.0, 90.0]
}
alice = {
"name": "Alice",
"homework": [100.0, 92.0, 98.0, 100.0],
"quizzes": [82.0, 83.0, 91.0],
"tests": [89.0, 97.0]
}
tyler = {
"name": "Tyler",
"homework": [0.0, 87.0, 75.0, 22.0],
"quizzes": [0.0, 75.0, 78.0],
"tests": [100.0, 100.0]
}
def average(numbers):
total = sum(numbers)
total = float(total)
av = total/len(numbers)
return av
def get_average(student):
homework=average(student["homework"])
quizzes=average(student["quizzes"])
tests=average(student["tests"])
return 0.1 * homework + 0.3 * quizzes + 0.6 * tests
def get_letter_grade(score):
if score > 89:
return "A"
elif score > 79:
return "B"
elif score > 69:
return "C"
elif score > 59:
return "D"
else:
return "F"
get_letter_grade(get_average(lloyd))
def get_class_average(students):
results= []
for student in students:
get_average(student)
results = results.append(get_average(student))
return average(results)
```