Please explain what I am doing wrong with this code

# 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):
    return sum(len(v) for v in teachers.values())

def courses(teachers):
    output = []
    for value in teachers.values():
        output += value
    return output

def most_courses(teachers):
    max_count = 0
    rockstar = ""
    for teacher, course_list in teachers.items():
        if len(course_list) > max_count:
            max_count = len(course_list)
            rockstar = teacher
    return rockstar