Teacher Stats: 4 of 5 - My solution seems to work in workspaces but fails in the challenge. What am I doing wrong?

# 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):
    total_courses = 0
    for teacher in teachers:
        total_courses += len(teachers[teacher])
    return total_courses

def courses(courses):
  all_courses = []
  for teacher in courses:
    all_courses += courses[teacher]
  return all_courses

def most_courses(courses):
  max_count = 0
  for teacher in courses:
    if len(courses[teacher]) > max_count:
      max_count = len(courses[teacher])
      max_teacher = teacher
    return max_teacher