UIBackgroundTask throws error `no background task exists`

Question

An AVAudioPlayer is stopped in a background tasks when the app transitions to background:

func stop() {

    var backgroundTask: UIBackgroundTaskIdentifier = UIBackgroundTaskInvalid
    backgroundTask = UIApplication.shared.beginBackgroundTask() {

        UIApplication.shared.endBackgroundTask(backgroundTask)
        backgroundTask = UIBackgroundTaskInvalid
    }

    DispatchQueue.global(qos: .default).async {

        self.audioPlayer?.pause()

        UIApplication.shared.endBackgroundTask(backgroundTask)
        backgroundTask = UIBackgroundTaskInvalid // Symbolic breakpoint stops here
    }
}

The function above gets called by the view controller that contains the AVAudioPlayer when a UIApplicationDidEnterBackground notification is posted.

Still I get an error at line backgroundTask = UIBackgroundTaskInvalid:

// Can't endBackgroundTask: no background task exists with identifier 1f, or it may have already been ended. Break in UIApplicationEndBackgroundTaskError() to debug.`

When I remove the line I get no errors. I looked through similar questions on SO but they did not help.

What is causing this error?