```
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)/len(numbers)
return total
students = [lloyd, alice, tyler]
def get_average(students):
homework = average(students["homework"])
quizzes = average(students["quizzes"])
tests = average(students["tests"])
grade1 = homework * 0.10
grade2 = tests * 0.60
grade3 = quizzes * 0.30
return grade1 + grade2 + grade3
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"
score = get_average(lloyd)
value = get_letter_grade(score)
print value
def get_class_average(students):
results = []
for student in students:
avgofstd= get_average(student)
results=results.append(avgofstd)
std=average(results)
return std
```

The error I get is

"Oops, try again. get_class_average([alice]) resulted in an error: 'NoneType' object is not iterable".What does this mean? and how do I correct it?