Vấn đề gặp phải khi thực hiện bài 25 learnpythonthehardway

Hi anh chị,
Mình chạy kết quả bài ex25 nhưng cứ báo lỗi như zầy, anh chị giúp mình lỗi này sửa sao
EX25:

def break_words(stuff):
	"""This fuction will break up words for us"""
	words = stuff.split(' ')
	return words

def sort_words(words):
	"""Sorts the words"""
	return sorted (words)

def print_first_word(words):
	"""Print the first word after popping it off"""
	word = words.pop(0)
	print word

def print_last_words(words):
	"""Print the first word after popping it off"""
	word = words.pop(-1)
	print word

def sort_sentence(sentence):
	"""Take in a full sentence and return the sorted words"""
	words = break_words (sentence)
	return sort_words (words)

def print_first_and_last(sentence):
	"""Print the first and last words of the sentence"""
	words = break_words(sentence)
	print_first_word(words)
	print_last_word(words)

def print_first_and_last_sorted(sentence):
	"""Sort the words then prints the first and last one"""
	words = sort_sentence (sentence)
	print_first_word(words)
	print_last_word(words)

Run

>>> import ex25
>>> sentence ="All good things come to those who wait."
>>> print sentence
All good things come to those who wait.
>>> words = ex25.break_words(sentence)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "ex25.py", line 3, in break_words
    words = stuff.split(' ')
**ValueError: empty separator**