what does "Cache values must be persistent objects" error mean ?

Hi,

When I open my old database in new application, I get following error. Does 
anybody have any idea about the meaning of this error ?

*------                                                                     
                                        *
*2016-09-19T13:03:32 ERROR ZODB.Connection Couldn't load state for 
BTrees.OIBTree.OIBTree 0x0fdeae2b                *
*Traceback (most recent call last):                                         
                                        *
*  File 
"C:\Python27\lib\site-packages\zodb-4.4.3-py2.7.egg\ZODB\Connection.py", 
line 899, in setstate              *
*    self._setstate(obj, oid)                                               
                                        *
*  File 
"C:\Python27\lib\site-packages\zodb-4.4.3-py2.7.egg\ZODB\Connection.py", 
line 956, in _setstate             *
*    self._reader.setGhostState(obj, p)                                     
                                        *
*  File 
"C:\Python27\lib\site-packages\zodb-4.4.3-py2.7.egg\ZODB\serialize.py", 
line 622, in setGhostState          *
*    state = self.getState(pickle)                                         
                                         *
*  File 
"C:\Python27\lib\site-packages\zodb-4.4.3-py2.7.egg\ZODB\serialize.py", 
line 615, in getState               *
*    return unpickler.load()                                               
                                         *
*  File 
"C:\Python27\lib\site-packages\zodb-4.4.3-py2.7.egg\ZODB\serialize.py", 
line 481, in _persistent_load       *
*    return self.load_persistent(*reference)                               
                                         *
*  File 
"C:\Python27\lib\site-packages\zodb-4.4.3-py2.7.egg\ZODB\serialize.py", 
line 523, in load_persistent        *
*    self._cache.new_ghost(oid, obj)                                       
                                         *
*TypeError: Cache values must be persistent objects.                    *   
                                         
 

How to catch the problematic area ?

Regards,
Sanjay

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