Can not turn off photo library backup from iCloud on 6s

I have an iPhone 6s with a lot of pictures and I want to remove my photos from backing up. Unfortunately, I can not remove or turn off the photo library from backing up. Whenever I try, I receive this message:

"Cannot Turn Off Backup" "Backups for Photo Library cannot be turned off at this time".

Is there something I'm not doing right or am I missing a step?