I am loading JSON in background thread and saving it in Core Data. 99.9% of times everything works, but occasionally I get a strange background crash, with this exception: [__NSCFString objectID]: unrecognized selector sent to instance 0x7b14001efcd0 ( 0   CoreFoundation                      0x00007fff2043a126 __exceptionPreprocess + 242 1   libobjc.A.dylib                     0x00007fff20177f78 objc_exception_throw + 48 2   CoreFoundation                      0x00007fff20448c6f +[NSObject(NSObject) instanceMethodSignatureForSelector:] + 0 3   CoreFoundation                      0x00007fff2043e666 _forwarding_ + 1489 4   CoreFoundation                      0x00007fff20440698 _CF_forwarding_prep_0 + 120 5   CoreData                            0x00007fff251a034b -[NSManagedObjectContext _disposeObjects:count:notifyParent:] + 176 6   CoreData                            0x00007fff25195b20 -[NSManagedObjectContext _dispose:] + 297 7   CoreData                            0x00007fff2510f004 -[NSManagedObjectContext _dealloc__] + 570 8   CoreData                            0x00007fff251ac1be __internalBlockToDeallocNSManagedObjectContext_block_invoke + 43 9   CoreData                            0x00007fff25109933 developerSubmittedBlockToNSManagedObjectContextPerform + 154 10  libclang_rt.tsan_iossim_dynamic.dyl 0x00000001098f2c72 _ZN6__tsanL22dispatch_callback_wrapEPv + 306 11  libdispatch.dylib                   0x00007fff20106534 _dispatch_client_callout + 8 12  libdispatch.dylib                   0x00007fff2010c425 _dispatch_lane_serial_drain + 715 13  libdispatch.dylib                   0x00007fff2010cfa0 _dispatch_lane_invoke + 403 14  libdispatch.dylib                   0x00007fff20117591 _dispatch_workloop_worker_thread + 782 15  libsystem_pthread.dylib             0x00007fff5dcd8a3d _pthread_wqthread + 290 16  libsystem_pthread.dylib             0x00007fff5dcd7b77 start_wqthread + 15 ) Object 0x7b14001efcd0 which is being asked for objectID it really is a string, it's actually the NSManagedObjectID: (lldb) po 0x7b14001efcd0 x-coredata://EA951A1D-C5F1-4CAA-80DA-D187C94817D4/Event/p101 Is this maybe a bug in Core Data or is there something I can do to fix this? Not a single line in my code is present in thread stack list.