[Sieve] Unable to fileinto subfolders

[james at lottspot.com]

Hello again list!

I am configuring a server running Dovecot 2.2.15 (Pigeonhole 0.4.6), and 
I am having an issue with Sieve scripts where I cannot sort messages 
into a subfolder of a mailbox. An email which is matched by the 
following script:

require "fileinto";
if header :contains "Subject" "[SORT ME]" { fileinto "INBOX.Sorted"; }

Yields the following error:

Jul 16 18:54:46 smtp dovecot: lmtp(james at lottspot.vpn): Error: 
DiJmNGZgqFWLFAEANWXNPw: sieve: msgid=<4973528.yvy4Yu8Tt4 at arch_project>: 
failed to store into mailbox 'INBOX.Sorted': Character not allowed in 
mailbox name: '.'

As far as I can tell based on the Sieve usage page in the wiki 
(http://wiki2.dovecot.org/Pigeonhole/Sieve/Usage), this is *supposed* to 
work. I have included my namespace configuration below, but if anyone 
wants the whole doveconf -n, I will gladly provide it. Thanks in advance 
for any help!

namespace inbox {
   type = private
   separator = /
   prefix =
   inbox = yes
   mailbox Drafts {
     auto = subscribe
     special_use = \Drafts
   }
   mailbox Sent {
     auto = subscribe
     special_use = \Sent
   }
   mailbox Spam {
     auto = subscribe
     special_use = \Junk
   }
   mailbox Trash {
     auto = subscribe
     special_use = \Trash
   }
}
namespace {
   type = shared
   separator = /
   prefix = shared/%%d/%%n/
   location = 
maildir:/home/%%d/%%n/mail:INDEX=~/indexes/%%d/%%n:INDEXPVT=~/indexes/%%d/%%n
   subscriptions = no
   list = children
}