Debian :: Backup Purge Script - `/mnt/backup/subfolder': No Such File Or Directory

This script simply deletes files older than a certain age (in this case 7 days) from a certain location; I use it to purge old backups nightly, and it works as expected:

# delete backups older than 7 days
find /mnt/backup/*  -mtime +7 -exec rm -Rf {} ;

The problem is, every morning I get an email with an error message something like this:

find: `/mnt/backup/subfolder': No such file or directory