Code works in local environment, does not work in challenge

# 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(teachers):
    return len(teachers)

def num_courses(teachers):
    courses = 0
    for teacher in teachers:
        courses += len(teachers[teacher])
    return courses

def courses(teachers):
    course_list = []
    for teacher in teachers:
        course_list.extend(teachers[teacher])
    return course_list

def most_courses(teachers):
    return max(teachers, key = lambda x: len(set(teachers[x])))

def stats(teachers):
    stat_list = []
    for teacher in teachers:
        teacher_list = []
        teacher_list.append(teacher)
        teacher_list.append(len(teachers[teacher]))
        stat_list.append(teacher_list)
    return stat_list