PYTHON归并排序，求排错！

def merge(left,right):
result = []
i,j = 0, 0
while i < len(left) and j < len(right):
if left[i] <= right[j]:
result.append(left[i])
i = i + 1
else:
result.append(right[j])
j = j + 1
while (i < len(left)):
result.append(left[i])
i = i + 1
while (j < len(right)):
result.append(right[j])
j = j + 1
return(result)

def mergsort(L):
print(L)
if len(L) < 2:
print(L[:])
else:
middle = len(L)/2
left = mergsort(L[:int(middle)])
right = mergsort(L[int(middle):])
together = merge(left,right)
print(together)
return(together)

错误信息：
Traceback (most recent call last):
File "", line 1, in
mergsort(M)
File "C:/Python33/bobble sort.py", line 62, in mergsort
left = mergsort(L[:int(middle)])
File "C:/Python33/bobble sort.py", line 62, in mergsort
left = mergsort(L[:int(middle)])
File "C:/Python33/bobble sort.py", line 64, in mergsort
together = merge(left,right)
File "C:/Python33/bobble sort.py", line 39, in merge
while i < len(left) and j < len(right):
TypeError: object of type 'NoneType' has no len()

mergort 有问题 不知道怎么能一直分割 跪求各位解答！

def merge(left,right): 
    result = [] 
    i,j = 0, 0 
    while i < len(left) and j < len(right):
        if left[i] <= right[j]:
            result.append(left[i]) 
            i = i + 1 
        else:
            result.append(right[j]) 
            j = j + 1
    while (i < len(left)):
        result.append(left[i]) 
        i = i + 1 
    while (j < len(right)):
        result.append(right[j]) 
        j = j + 1 
    return(result)

def mergsort(L):
    print(L)
    if len(L) < 2:
        print(L[:])
        # Missing the following line
        return L
    else:
        middle = len(L)/2
        left = mergsort(L[:int(middle)])
        right = mergsort(L[int(middle):])
        together = merge(left,right)
        print(together)
        return(together)

期待看到有用的回答！