[Dovecot] Virtual Folders

I have the following setup:

dovecot --version 1.2.10

namespace private { prefix = virtual. separator = . location = virtual:~/Maildir/virtual:INDEX=MEMORY }

protocol imap { mail_plugins = quota imap_quota fts fts_solr antispam virtual

#pwd ~/Maildir/virtual/all

#cat dovecot-virtual * all

The problem is that I can't access any of the virtual folders. Logging
the IMAP conversation showed me this:

5ptq SELECT "virtual.all" frdv IDLE 5ptq NO Mailbox doesn't exist: all (all)^M

• idling^M frdv OK Idle completed.^M

What am I doing wrong ?