Back up error in"Keep"

Has anyone got any ideas of the following error when using the back up now option in "Keep"

An error occured making /home/mark/music backup:
Fatal Error: Destination directory /home/mark/Desktop exists, but does not look like a rdiff-backup directory. Running rdiff-backup like this could mess up what is currently in it. If you want to update or overwrite it, run rdiff-backup with the --force option.

Thanks