15.0-RELEASE boot panic when if_iwm_load="YES"

I know 15.0 is not released yet but the image is up and I wanted to try it
out on real hardware.

After I put if_iwm_load="YES" in /boot/loader.conf it fails to boot.
After saying it's loading loader.conf and briefly flashing FreeBSD logo I
get a panic:

panic: page fault
cpuid = 0
time = 1
KDB: stack backtrace:
#0 0xffffffff80bbe1ed at kdb_backtrace+0x5d
#1 0xffffffff80b71576 at vpanic+0x136
#2 0xffffffff80b71433 at panic+0x43
#3 0xffffffff81079f69 at trap_pfault+0x3c9
#4 0xffffffff8104ff18 at calltrap+0x8
#5 0xffffffff80c4f974 at namei+0xe4
#6 0xffffffff80c768b2 at vn_open_cred+0x5b2
#7 0xffffffff80bb9ef4 at loadimage+0x274
#8 0xffffffff80bd4de2 at taskqueue_run_locked+0x182
#9 0xffffffff80bd5fb2 at taskqueue_thread_loop+0xc2
#10 0xffffffff80b2786b at fork_exit+0x7b
#11 0xffffffff81050f3e at fork_trampoline+0xe
Uptime: 1s
Automatic reboot in 15 seconds - press a key on the console to abort

Am I doing something wrong?
IIRC this worked on 14.3 on the same hardware.