ios8 - Swift: what does swift_retainUnowned mean? -

i crashlogs following:

thread : crashed: com.apple.main-thread 0  libswiftcore.dylib             0x00000001006e6c78 swift_retainunowned + 50 1  libswiftcore.dylib             0x00000001006f2b3c swift_unknownretainunowned + 316 

that crash seems appear here

self.custompicker.cancelblock = {[unowned self] in     self.setcustompickervisibility(show:false, textfield:nil) } 

i have no idea means or how can happen. assume self can't deallocated @ point, should safe call closure.