hammer crash on cleanups

[Eugene <dfuser@xxxxxxxxxxxxx>]

Hello All.
I've got a time-to-time reproducible panic while running cleanups on a 
mirror built with hammer mirror-stream.
I'm running a system built on -DEVELOPMENT sources from August, 9th 
(uses SILI device) and a following system layout:

/dev/da0s1a      /          ufs     rw  1  1
/dev/da0s2b      none       swap    sw  0  0
/dev/da0s2h      /HAMMER0   hammer  rw  2  2
/HAMMER0/pfs/var /var       null    rw  2  2
/dev/da1s1a      /mirror    ufs     rw  1  1 # not used
/dev/da1s2b      none       swap    sw  0  0
/dev/da1s2h      /HAMMER1   hammer  rw  2  2
/HAMMER1/pfs/var /mirrorvar null    rw  2  2
proc             /proc      procfs  rw  0  0

A kernel panic occurs on running daily cleanups on a slave part of 
mirror and I managed to reproduce it manually while executing %hammer 
reblock and %hammer rebalance commands on a slave pfs after a day or two 
of uptime.
When running %hammer cleanup immediately after system boot-up, it always 
runs fine and resumes with no error.
I've attached a screenshot of a latest panic I've got while running cleanup.

If there can be any solution for this problem?

--
Sorry for my poor English.