Custom cache_key for passdb-sql

The cache_key for the SQL-passdb should be created automatically, but for some reason the service (%s) is not part of the key, and I need it to be

I've tried to set a manual cache_key, but that fails:

Mar 15 11:54:02 auth: Fatal: sql /etc/dovecot/dovecot-sql.conf cache_key=%s%u%{real_ip}: Can't open configuration file /etc/dovecot/dovecot-sql.conf cache_key=%s%u%{real_ip}: No such file or directory

passdb { args = /etc/dovecot/dovecot-sql.conf cache_key=%s%u%{real_ip} driver = sql }

Is this not possible?

-- Tom