teachers.py Task 4. Why is teachers results not accepted?

# 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(arg):
    num_teachers=len(arg)
    return num_teachers

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

def courses(arg):
    list_of_courses=[]
    for teachers in arg:
        list_of_courses += arg[teachers]
    return list_of_courses

def most_courses(arg):
    most_courses_num = 0
    num_of_courses = 0
    for teacher in arg:
        num_of_courses=len(arg[teacher])
        if num_of_courses > most_courses_num:
            most_courses_teacher=[]
            most_courses_teacher.append(teacher)
            most_courses_num=num_of_courses
        elif num_of_courses == most_courses_num:
            most_courses_teacher.append(teacher)
        else:
            None
    return most_courses_teacher