This fixes the following an invalidation bug:

ERROR ZODB.Connection Couldn't load state for 0x504e
Traceback (most recent call last):
  File "ZODB/Connection.py", line 851, in setstate
    self._setstate(obj)
  File "ZODB/Connection.py", line 916, in _setstate
    self._load_before_or_conflict(obj)
  File "ZODB/Connection.py", line 931, in _load_before_or_conflict
    if not self._setstate_noncurrent(obj):
  File "ZODB/Connection.py", line 954, in _setstate_noncurrent
    assert end is not None
AssertionError