[pgbackrest] Expiring the last backup?

Hi,

The backups partition is running out of disk space, and I need to delete the 
only backup.

$ pgbackrest expire --stanza=localhost --retention-full=0
ERROR: [032]: '0' is out of range for 'repo1-retention-full' option

So, in version 2.07, what's the secret sauce for deleting that final copy?

(Yes, we're getting more disk space, but that takes time.)

-- 
Angular momentum makes the world go 'round.