What's wrong with my solution?

# 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(some_dict):
    return len(some_dict.keys())

def num_courses(some_dict):
    return sum(len(courses) for courses in some_dict.values())

def courses(some_dict):
    return [i for courses in some_dict.values() for i in courses]

def most_courses(some_dict):
    max_key = None
    big = 0
    for k, v in some_dict.items():
        if len(v) > big:
            big = len(v)
            max_key = k
    return max_key'

def stats(some_dict):
    return [[k, len(v)] for k, v in some_dict.items()]