last function throwing an error

# 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(dic):
    count=0
    for x in dic.keys():
        count+=1
    return count    
def num_courses(dic):
    count=0
    for x in dic.values():
        count+=len(x)
    return count   
def courses(dic):
    lisst=[]
    for xx in dic.values():
        for x in xx:
            lisst.append(x)

    return lisst       
def most_courses(dic):
    max_count=0
    winner=""
    for x in dic.keys():
        count=len(dic[x])
        if count >= max_count:
            max_count=count
            winner=x
    return winner        

def stats(dic):
    small=[]
    big=[]
    for x in dic.keys():
        small.append(x)
        y=len(dic[x])
        small.append(y)
        big.append(small) 
        small=[]
    return big