It return the correct answer in IDE but does not pass the challenge, is it because it also returns an None?

# The dictionary will look something like:
# {'Andrew Chalkley': ['jQuery Basics', 'Node.js Basics'],
#  'Kenneth Love': ['Python Basics', 'Python Collections']}
#
# Each key will be a Teacher and the value will be a list of courses.
#
# Your code goes below here.
def num_teachers(d):
    number_of_teachers = len(d.keys())
    return number_of_teachers

def num_courses(d):
    n = 0
    for key in d:
        n = n + len(d[key])
    return n

def courses(d):
    l = []
    for key in d:
        l = l + d[key][0:]
    return l


def most_courses (d):
    largest = max(d.values(), key=len)
    for key in d:
        if d[key] == largest:
            print key