random failure

>>> random.choice({0:1, 2:3})
1
>>> random.choice({0:1, 2:3})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python27\lib\random.py", line 274, in choice
    return seq[int(self.random() * len(seq))]  # raises IndexError if seq is empty 
KeyError: 1

Alright pythonauts, why does this fail sometimes and succeed others? :-)