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

```
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]
}
# Add your function below!
def average(numbers):
total = sum(numbers)
total = float(total)
a = total / len(numbers)
return a
def get_average(student):
homework = average(student['homework'])
quizzes = average(student['quizzes'])
tests = average(student['tests'])
return homework * 0.1 + quizzes * 0.3 + tests* 0.6
def get_letter_grade(score):
if score >= 90:
return 'A'
elif score >= 80:
return 'B'
elif score >= 70:
return 'C'
elif score >= 60:
return 'D'
else:
return 'F'
def get_class_average(students):
results=[]
for student in students:
a = get_average(student)
results = results.append(a)
return average(results)
```