zfs crash on FreeBSD 10.3

I attempted to add a second partition to an existing FS pool in 
FreeBSD 10.3 and the result was a crash..

is there anyone out there with a scratch system (10.3) (or two spare 
drives) who can show me this working?

Does it look familiar to anyone?

The drive 'boot0' is being used as the root drive, but we are in 
single user mode, so its' read-only at this stage.

=============== cut-n-paste =============

[boot -s]

[...]

Trying to mount root from zfs:p8/image1 []...
Enter full pathname of shell or RETURN for /bin/sh:
PS1="# "
#
#
# ls /dev/gpt
boot0    boot1
# zpool attach -f p8 gpt/boot0 gpt/boot1

Fatal trap 12: page fault while in kernel mode
cpuid = 1; apic id = 01
fault virtual address    = 0x50
fault code        = supervisor read data, page not present
instruction pointer    = 0x20:0xffffffff81717063
stack pointer            = 0x28:0xfffffe0169bfc640
frame pointer            = 0x28:0xfffffe0169bfc9a0
code segment        = base 0x0, limit 0xfffff, type 0x1b
             = DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags    = interrupt enabled, resume, IOPL = 0
current process        = 3 (txg_thread_enter)
trap number        = 12
Panic:Thought about setting the watchdog to 10 Minutes
panic: page fault
cpuid = 1

KDB: stack backtrace:
  stack1 db_trace_self_wrapper+0x2a
  kdb_backtrace+0x37 vpanic+0xf7
  panic+0x67 trap_fatal+0x264
  trap_pfault+0x1c2
  trap+0x38c
  calltrap+0x8
  dsl_scan_sync+0x2f3
  spa_sync+0x328
  txg_sync_thread+0x140
  fork_exit+0x135
  fork_trampoline+0xe

KDB: enter: panic
[ thread pid 3 tid 100328 ]
Stopped at      kdb_enter+0x50: movq    $0,0x6bd5cd(%rip)
db> reboot

I will add that after this, every boot hits this problem. (same stack 
trace). the box is effectively bricked
(or would be if it weren't just a bhyve image)