[ZODB-Dev] What causes a persistent object to be "broken"?

[kw]

I'm using a PersistentMapping and have noticed that
one of the mappings shows the following, after
creating and committing a one item mapping:

{u'test_system': <persistent broken __main__.System
instance '\x00\x00\x00\x00\x00\x00\x00\x1a'>}

Here __main__.System is a custom datatype that's a
composite of smaller objects, that are themselves
persistent.  In general, why does a persistent object
become "broken"?   What needs to be done to fix this? 
Any help would be appreciated,

kw_odonian