Setting lmtp_user_concurrency_limit causes anvil permission error

I've set lmtp_user_concurrency_limit to 5 and now LMTP throws this at me for every delivery:

Apr 07 13:38:33 lmtp(4434): Error: net_connect_unix(/var/run/dovecot/anvil) failed: Permission denied

ls -l /var/run/dovecot/anvil srw------- 1 root root 0 Apr 7 13:32 /var/run/dovecot/anvil

If I set lmtp_user_concurrency_limit to 0, the error goes away.

-- Tom