[Dovecot] Dovecot 1.0.15, sieve fileinto subdirectory ?

Hi, does sieve plugin in 1.0.15 support fileinto subdirectory?

There is a maildir .oss.dovecot and sieve rule:

if envelope :contains "to" "k+dovecot" { fileinto "oss.dovecot"; }

But I get LDA error:

Couldn't open mailbox dovecot: Mailbox doesn't exist: dovecot

Any suggestions? Delivery to non-subdirectories is working fine.

Regards

Karel