Can't pickle bare instructions

>>> import loopy, pickle
>>> insn = loopy.Assignment("var", "val")
>>> pickle.loads(pickle.dumps(insn))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/matt/src/loopy/loopy/kernel/instruction.py", line 443, in __setstate__
    self.id = intern(self.id)
TypeError: must be str, not None