PHPhotoLibraryChangeObserver doesn't work when I take a screenshot

I have a gallery that shows every photo/video from my camera roll in fullscreen.
I added a PHPhotoLibraryChangeObserver to my app, so if something changes in the library it should also change in my gallery. You can navigate through the photos/videos with sliding.
If I exit my app (pressing the home button not manually close it), take a photo with the camera and open it again everything works fine.
Now my problem is that if I take a screenshot, the camera roll also changes and then the program doesn't work well anymore.

This code I use for observering the camera roll:

PHPhotoLibrary.sharedPhotoLibrary().registerChangeObserver(self) and this function:

func photoLibraryDidChange(changeInstance: PHChange) {
    dispatch_async(dispatch_get_main_queue(), {
        self.updateFetchResult()
    })

}