I have an application that downloads/parses a big XML file and store the information using core data (approx. 4000 objects (entities)). The XML is loaded/parsed in a different thread, which has its own NSManagedObjectContext. When trying to save the entities to the persistent store, I sometimes get the following error (about 20%) 2010-03-03 23:41:42.802 xxx[7487:4203] Exception in XML saving 2010-03-03 23:41:42.802 xxx[7487:4203] Description: * -_referenceData64 only defined for abstract class. Define -[NSTemporaryObjectID_default _referenceData64]! 2010-03-03 23:41:42.803 xxx[7487:4203] Name: NSInvalidArgumentException 2010-03-03 23:41:42.804 xxx[7487:4203] UserInfo: (null) 2010-03-03 23:41:42.805 xxx[7487:4203] Reason: * -_referenceData64 only defined for abstract class. Define -[NSTemporaryObjectID_default _referenceData64]! I have a simple integer to keep track of the entities the application creates compared to the insertedObjects property in the NSManagedObjectContext before saving, and when I get the error, these numbers do not match, insertedObjects in the NSManagedObjectContext is missing about 10 entities. I do not know how I should continue to investigate this problem, anyone has any idea how to fix this? Thanks /oscar