panic: sleepq_broadcast: invalid NULL wait channel

6.0 from a couple of days ago, panicked at reboot:

Waiting (max 60 seconds) for system process `vnlru' to stop...done
Waiting (max 60 seconds) for system process `bufdaemon' to stop...done
Waiting (max 60 seconds) for system process `syncer' to stop...
Syncing disks, vnodes remaining...6 4 3 1 1 0 0 done
All buffers synced.
unmount of /dev failed (BUSY)
Uptime: 14m9s
panic: sleepq_broadcast: invalid NULL wait channel
cpuid = 0
KDB: enter: panic
[thread pid 5125 tid 100145 ]
Stopped at      kdb_enter+0x31: leave
db> wh
Tracing pid 5125 tid 100145 td 0xffffff0045f67980
kdb_enter() at kdb_enter+0x31
panic() at panic+0x1e6
sleepq_broadcast() at sleepq_broadcast+0x41
wakeup() at wakeup+0x23
arcmsr_shutdown() at arcmsr_shutdown+0x19a
device_shutdown() at device_shutdown+0x47
bus_generic_shutdown() at bus_generic_shutdown+0x1a
device_shutdown() at device_shutdown+0x47
bus_generic_shutdown() at bus_generic_shutdown+0x1a
device_shutdown() at device_shutdown+0x47
bus_generic_shutdown() at bus_generic_shutdown+0x1a
device_shutdown() at device_shutdown+0x47
bus_generic_shutdown() at bus_generic_shutdown+0x1a
device_shutdown() at device_shutdown+0x47
bus_generic_shutdown() at bus_generic_shutdown+0x1a
device_shutdown() at device_shutdown+0x47
bus_generic_shutdown() at bus_generic_shutdown+0x1a
device_shutdown() at device_shutdown+0x47
bus_generic_shutdown() at bus_generic_shutdown+0x1a
acpi_shutdown() at acpi_shutdown+0x31
device_shutdown() at device_shutdown+0x47
bus_generic_shutdown() at bus_generic_shutdown+0x1a
device_shutdown() at device_shutdown+0x47
bus_generic_shutdown() at bus_generic_shutdown+0x1a
device_shutdown() at device_shutdown+0x47
root_bus_module_handler() at root_bus_module_handler+0xc2
module_shutdown() at module_shutdown+0x42
boot() at boot+0x6a3
reboot() at reboot+0x39
syscall() at syscall+0x320
Xfast_syscall() at Xfast_syscall+0xa8
--- syscall (55, FreeBSD ELF64, reboot), rip = 0x80076abcc, rsp = 0x7fffffffeb08, rbp = 0 ---
db>