CURRENT: md: g_handleattr: md0 bio_length 24 len 31 -> EFAULT

While writing out a md based disk image (NanoBSD) onto a ZFS based filesystem,
the console is flooded on CURRENT host (FreeBSD 12.0-CURRENT #78 r330019: Mon
Feb 26 16:23:09 CET 2018 amd64) with the message:

g_handleattr: md0 bio_length 24 len 31 -> EFAULT

Writing out the md image of a size of ~ 2,5 GB to ZFS filesystem takes more
than 20 minutes, which is a pain in the arse. ZFS hasn't any neat features
enabled or disabled, so deduplication is disabled, atime is off, checksum is
sha256 and compression is disabled. The box in question is a Haswell XEON, 6
cores, 3,4 GHz clock and 32 GB RAM - so insufficient performance should not be
the issue. 

What am I doing wrong?

Kind regards and thanks in advance,

Oliver