Cannot use relative paths in 'Backup Location'

When entering the Backup Location on /admin/content/backup, I read the description under the field 'The directory where the backup will be written. This is relative to Drupal's root directory (/home/matt/workspace/www/html). If not specified, the default will be Drupal's root directory.' to mean that I can specify a relative path to the roor directory,

I have created a writeable folder, one level up from the root directory. If I enter '../backups' and Save Config, I get the message "Path '' is not writable. Note that the user the webserver runs as may be different from the user that owns that directory. Please contact your SysAdmin for further assistance."

However, all is ok if I put in the full path /home/matt/workspace/www/backups

Either the descriptive text under the Backup Location field is not correct, or the module cannot handle relative paths and should.