Task parameter is nil in didCompleteWithError callback

Question

I'm trying to upload files using NSURLSessionUploadTasks and a background NSURLSession. The problem is that when the upload completes, the URLSession:task:didCompleteWithError: callback is invoked with a nil task parameter. The session parameter is not nil so no problem there. The task parameter is not nil in the URLSession:dataTask:didReceiveData: callback that is called prior to the completion callback. My test device is an iPhone 7 with iOS v11.0.