most_courses function gives me an error message

# 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(my_dict):
     return int(len(my_dict.keys()))



def num_courses(my_dict):
     counter = 0
     for value in my_dict.values():
         for lstValue in value:
             counter += 1

     return int(counter)

def courses(my_dict):
    course_list = []
    for item in my_dict.values():
        for v in item:
            course_list.append(v)
    return course_list

def most_courses(my_dict):
    counter = 0
    winner = ""
    for k, v in my_dict.items():
        if len(v) > counter:
            counter += len(v)
            winner = str(k)

    return winner