python collections.defaultdict()编译错误

import string
import collections

#create dictionary with alphabets as keys,and empty values
list = ['aema','airplane','amend']

gen_dict = dict.fromkeys(string.ascii_lowercase,'')

gen_dict = collections.defaultdict(list)

for x in list:
    gen_dict['a'].append(x)

并产生的错误是：

Traceback (most recent call last):
  File "indexdict.py",line 14,in <module>
    gen_dict = collections.defaultdict(list)
TypeError: first argument must be callable

任何的想法？
提前致谢