mdadm doesn't start array with external bitmap

hi, all!

i've just created a raid5 array with mdadm:
root@nas:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid5 sdf1[5] sde1[3] sdd1[2] sdc1[1] sdb1[0]
      7814047744 blocks super 1.2 level 5, 512k chunk, algorithm 2 [5/5] [UUUUU]
      bitmap: 0/466 pages [0KB], 2048KB chunk, file: /var/md0_wr_intent

unused devices: <none>

root@nas:~# mdadm --detail --scan
ARRAY /dev/md/0 metadata=1.2 bitmap=/var/md0_wr_intent name=nas:0 UUID=6b03bf85:5d6c5d4e:782f3370:ae7fbb89

root@nas:~# mdadm --detail --scan >> /etc/mdadm/mdadm.conf
root@nas:~# update-initramfs -k all -u

but after reboot i get te following:

root@nas:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : inactive sdf1[5](S) sde1[3](S) sdd1[2](S) sdc1[1](S) sdb1[0](S)
      9767562680 blocks super 1.2

unused devices: <none>

if i issue mdadm --stop /dev/md0 and mdadm --assemble --scan, it assembles as desired!

root@nas:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid5 sdb1[0] sdf1[5] sde1[3] sdd1[2] sdc1[1]
      7814047744 blocks super 1.2 level 5, 512k chunk, algorithm 2 [5/5] [UUUUU]
      bitmap: 0/466 pages [0KB], 2048KB chunk, file: /var/md0_wr_intent

unused devices: <none>

and if i remove bitmap= option from mdadm.conf, array starts normally on boot.
array also starts okay with auto detection (if i remove ARRAY line from mdadm.conf), but with internal bitmap
/var is located on rootfs, not a separate partition

root@nas:~# lsb_release -rd
Description: Ubuntu 11.04
Release: 11.04
root@nas:~# apt-cache policy mdadm
mdadm:
  Installed: 3.1.4-1+8efb9d1ubuntu4.1
  Candidate: 3.1.4-1+8efb9d1ubuntu4.1
  Version table:
 *** 3.1.4-1+8efb9d1ubuntu4.1 0
        500 http://ru.archive.ubuntu.com/ubuntu/ natty-updates/main i386 Packages
        100 /var/lib/dpkg/status
     3.1.4-1+8efb9d1ubuntu4 0
        500 http://ru.archive.ubuntu.com/ubuntu/ natty/main i386 Packages

any glue how to set external bitmap without breaking things?
Thanks in advance!