https://www.codecademy.com/courses/python-beginner-en-qzsCL/1/4?curriculum_id=4f89dab3d788890003000096

get_class_average([alice]) resulted in an error: unsupported operand type(s) for +: 'int' and '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]
}
# Add your function below!
def average(numbers):
total = sum(numbers)
total = float(total)
average = total / len(numbers)
return average
def get_average(student):
homework = average(student["homework"])
quizzes = average(student["quizzes"])
tests = average(student["tests"])
homework*0.1 + quizzes*0.3 + tests*0.6
def get_letter_grade(score) :
if score >= 90:
return "A"
elif score >= 80 and score < 90:
return "B"
elif score >= 70and score < 80:
return "C"
elif score >= 60and score < 70:
return "D"
else:
return "F"
def get_class_average(stud) :
results = []
for stu in stud:
results.append(get_average(stu))
return average(results)
```