Panic when loading vboxdrv module

I'm running 12-CURRENT r303286 and virtualbox-ose-kmod-5.0.26.  I just
rebuilt the module, so I know they're in sync.  When I boot with
vboxdrv_load="YES" in /etc/loader.conf, I get this panic during boot:

> panic: mutex Giant owned at /usr/src/sys/kern/kern_sync.c:406
> cpuid = 0
> KDB: stack backtrace:
> db_trace_self_wrapper() at db_trace_self_wrapper+0x2b
> vpanic() at vpanic+0x182
> panic() at panic+0x43
> __mtx_assert() at __mtx_assert+0xd1
> mi_switch() at mi_switch+0x7b
> sleepq_switch() at sleepq_switch+0x7e
> sleepq_timedwait() at sleepq_timedwait+0x43
> rtR0SemEventWait() at rtR0SemEventWait+0x267
> supdrvGipCreate() at supdrvGipCreate+0x7f6
> supdrvInitDexExt() at supdrvInitDexExt+0x18f
> VBoxDrvFreeBSDModuleEvent() at VBoxDrvFreeBSDModuleEvent+0x49
> module_register_init() at module_register_init+0xb0
> mi_startup() at mi_startup+0x118
> btext() at btext+0x2c
> KDB: enter: panic
> [ thread pid 0 tid 100000 ]
> Stopped at      kdb_enter+0x3b: movq    $0,kdb_why

I'm not sure whether this is a problem with the base system or the port,
but this happens every time I boot with the module enabled.