I'm getting a crash similar to PaulSarmale  Fatal Exception: NSInvalidArgumentException  0 CoreFoundation         0x9e88 __exceptionPreprocess  1 libobjc.A.dylib        0x178d8 objc_exception_throw  2 CoreFoundation         0x1af0a8 -[__NSCFString characterAtIndex:].cold.1  3 CoreFoundation         0x1abad0 -[__NSArrayM insertObject:atIndex:].cold.2  4 CoreFoundation         0x13f90 -[__NSArrayM insertObject:atIndex:]  5 AVFCore            0x17e58 __22-[AVPlayer _addLayer:]_block_invoke  6 libdispatch.dylib       0x3fdc _dispatch_client_callout  7 libdispatch.dylib       0x13574 _dispatch_lane_barrier_sync_invoke_and_complete  8 AVFCore            0x17548 -[AVPlayer _addLayer:]  9 AVFCore            0x17400 -[AVPlayerLayer _setPlayer:forPIP:]  10 Foundation           0x3f814 -[NSObject(NSKeyValueObservingPrivate) _changeValueForKeys:count:maybeOldValuesDict:maybeNewValuesDict:usingBlock:]  11 Foundation           0x3f590 -[NSObject(NSKeyValueObservingPrivate) _changeValueForKey:key:key:usingBlock:]  12 Foundation           0x3eb24 _NSSetObjectValueAndNotify  13 AVFCore            0x176bc -[AVPlayer _addLayer:]  14 AVFCore            0x17400 -[AVPlayerLayer _setPlayer:forPIP:]  15 Foundation           0x3f814 -[NSObject(NSKeyValueObservingPrivate) _changeValueForKeys:count:maybeOldValuesDict:maybeNewValuesDict:usingBlock:]  16 Foundation           0x3f590 -[NSObject(NSKeyValueObservingPrivate) _changeValueForKey:key:key:usingBlock:]  17 Foundation           0x3eb24 _NSSetObjectValueAndNotify  18 AVKit             0x21724 -[__AVPlayerLayerView setPlayerController:]  19 AVKit             0x1fdac -[AVPlayerViewController setPlayerController:]  20 Foundation           0x3f814 -[NSObject(NSKeyValueObservingPrivate) _changeValueForKeys:count:maybeOldValuesDict:maybeNewValuesDict:usingBlock:]  21 Foundation           0x3f590 -[NSObject(NSKeyValueObservingPrivate) _changeValueForKey:key:key:usingBlock:]  22 Foundation           0x3eb24 _NSSetObjectValueAndNotify  23 AVKit             0x307f4 -[AVPlayerViewController setPlayer:]  24 <APPNAME>           0x7b8418 closure #1 in AVPlayerBackgroundAudio.setupBecomeActiveSubscription() (<compiler-generated>)  25 Combine            0xb754 Subscribers.Sink.receive(_:)  26 Combine            0xb0a8 protocol witness for Subscriber.receive(_:) in conformance Subscribers.Sink<A, B>  27 Combine            0x12904 closure #1 in Publishers.ReceiveOn.Inner.receive(_:)  28 libswiftDispatch.dylib     0xf58 thunk for @escaping @callee_guaranteed () -> ()  29 libdispatch.dylib       0x24b4 _dispatch_call_block_and_release  30 libdispatch.dylib       0x3fdc _dispatch_client_callout  31 libdispatch.dylib       0x127f4 _dispatch_main_queue_drain  32 libdispatch.dylib       0x12444 _dispatch_main_queue_callback_4CF  33 CoreFoundation         0x9a6f8 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__  34 CoreFoundation         0x7c058 __CFRunLoopRun  35 CoreFoundation         0x80ed4 CFRunLoopRunSpecific  36 GraphicsServices        0x1368 GSEventRunModal  37 UIKitCore           0x3a23d0 -[UIApplication _run]  38 UIKitCore           0x3a2034 UIApplicationMain  39 <APPNAME>           0x7f74 main + 32 (AppDelegate.swift:32)  ) My code is trying to re-attach the player when coming back from the background: NotificationCenter.default.publisher(for: UIApplication.didBecomeActiveNotification)       .receive(on: DispatchQueue.main)       .sink { [weak self] _ in         guard let self else { return }                   if self.playerController?.player == nil && self.player != nil {           self.playerController?.player = self.player         }       }       .store(in: &cancellables) I even check that the player is not nil directly before the call. Also, it should be valid to assign it to nil anyways.