ipfw module panic on kldunload

When unloading ipfw module, I see this:

# kldunload ifpw
IP firewall unloaded
panic: mtx_lock() of destroyed mutex _at_
/usr/src/sys/modules/ipfw/../../netinet/ipfw/ip_fw2.c:3594
cpuid = 0
KDB: enter: panic
exclusive rw IPFW static rules (IPFW static rules) r = 0
(0xffffffff8104de18) locked _at_
/usr/src/sys/modules/ipfw/../../netinet/ipfw/ip_fw2.c:4754
exclusive sleep mutex Giant (Giant) r = 0 (0xffffffff80798700) locked
_at_ /usr/src/sys/kern/kern_linker.c:274

...backtrace from ddb.txt

db:0:kdb.enter.panic>  bt
Tracing pid 1149 tid 100051 td 0xffffff000188c720
kdb_enter() at kdb_enter+0x3d
panic() at panic+0x17b
_mtx_lock_flags() at _mtx_lock_flags+0x135
remove_rule() at remove_rule+0x47
free_chain() at free_chain+0x3b
vnet_ipfw_uninit() at vnet_ipfw_uninit+0x69
linker_file_unload() at linker_file_unload+0x4a6
kern_kldunload() at kern_kldunload+0xf3
syscall() at syscall+0x1dd
Xfast_syscall() at Xfast_syscall+0xe1
--- syscall (444, FreeBSD ELF64, kldunloadf), rip = 0x80069ceec, rsp =
0x7fffffffe328, rbp = 0xc ---

I can provide a textdump if required.

-Brandon