most_courses challenge: My code works on my system but not on treehouse, can anybody help

# 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(mydict):
    i=0
    for x in mydict:
        i+=1
    return i

def num_courses(mydict):
    i=0
    for values in mydict.values():
        for x in values:
            i+=1
    return i

def courses(mydict):
    mylist = []
    for values in mydict.values():
        for x in values:
            mylist.append(x)
    return mylist    

def most_courses(mydict):
    mc = 0
    for valueM in mydict.values():
        i = 0
        for x in valueM:
            i += 1
        if i > mc:
            mc = i
            for key, value in mydict.iteritems():
                if value == valueM:
                    teacher = key
    return teacher