Panic in unionfs "it is not a unionfs-vnode"

Hi,

I got the panic below in unionfs. Head as at 325569, 2017-11-09 12:41:00 +1100 (Thu, 09 Nov 2017).

This is a ufs filesystem union mounted on top of a read-only ufs /etc. I know unionfs has "architecture issues”. Is this resolvable? 

panic: unionfs: it is not unionfs-vnode
cpuid = 5
time = 1510678800
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe02c418c280
vpanic() at vpanic+0x19c/frame 0xfffffe02c418c300
kassert_panic() at kassert_panic+0x126/frame 0xfffffe02c418c370
unionfs_lock() at unionfs_lock+0x635/frame 0xfffffe02c418c400
VOP_LOCK1_APV() at VOP_LOCK1_APV+0xd9/frame 0xfffffe02c418c430
_vn_lock() at _vn_lock+0x66/frame 0xfffffe02c418c4a0
vget() at vget+0x82/frame 0xfffffe02c418c4e0
cache_lookup() at cache_lookup+0x787/frame 0xfffffe02c418c5c0
vfs_cache_lookup() at vfs_cache_lookup+0xac/frame 0xfffffe02c418c620
VOP_LOOKUP_APV() at VOP_LOOKUP_APV+0xd3/frame 0xfffffe02c418c650
lookup() at lookup+0x682/frame 0xfffffe02c418c6f0
namei() at namei+0x51a/frame 0xfffffe02c418c7b0
kern_statat() at kern_statat+0x98/frame 0xfffffe02c418c9c0
sys_fstatat() at sys_fstatat+0x2f/frame 0xfffffe02c418cac0
amd64_syscall() at amd64_syscall+0x79b/frame 0xfffffe02c418cbf0
Xfast_syscall() at Xfast_syscall+0xfb/frame 0xfffffe02c418cbf0
--- syscall (552, FreeBSD ELF64, sys_fstatat), rip = 0x800dd48da, rsp = 0x7fffffffddc8, rbp = 0x7fffffffde70 —


Thanks,

Jan Mikkelsen