Why is the last function incorrect?

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

def num_courses(dictionary):
    courses=0
    for course in dictionary.values():
        courses+=len(course)
    return courses

def courses(dictionary):
    av_courses=[]
    for courses in dictionary.values():
        av_courses.extend(courses)
    return av_courses

def most_courses(dictionary):
    max_count = 0
    MVP = ""
    for key, values in dictionary.item():
        if len(values) > max_count:
            max_count= len(values)
            MVP = keys
    return MVP