[Dovecot] Invalid quota rule using "backend"

[Laurento Frittella]

Using a quota rule like "quota_rule = *:backend=1073741824S" (I'm
testing it statically but the value is on LDAP) I receive an an error:

Fatal: IMAP(...): Quota root QUOTA: Invalid rule: *:backend=1073741824S

Accordingly with the wiki page http://wiki.dovecot.org/Quota/New
"Backend-specific configuration currently is used only with Maildir++
quota backend. It means you can have the quota in Maildir++ format (e.g.
"10000000S")." so I think it should work.

What's wrong?

Cheers,
Laurento