iwm load panic

I get this panic periodically at iwm load time:

(kgdb) p ic->ic_tq
value has been optimized out
(kgdb) down
#12 taskqueue_drain (queue=0x0, task=0xfffffe004fc17150) at
/usr/home/mmacy/drm-next-4.6/sys/kern/subr_taskqueue.c:554
554             TQ_LOCK(queue);
(kgdb) bt
#0  __curthread () at ./machine/pcpu.h:221
#1  doadump (textdump=0) at
/usr/home/mmacy/drm-next-4.6/sys/kern/kern_shutdown.c:298
#2  0xffffffff82703ac9 in vt_kms_postswitch (arg=<optimized out>)
    at /usr/home/mmacy/drm-next-4.6/sys/modules/drm2/drm2/../../../dev/drm2/linux_fb.c:82
#3  0xffffffff8093fb55 in vt_window_switch (vw=0xffffffff817e87d0
<vt_conswindow>)
    at /usr/home/mmacy/drm-next-4.6/sys/dev/vt/vt_core.c:540
#4  0xffffffff8093c9b0 in vtterm_cngrab (tm=<optimized out>) at
/usr/home/mmacy/drm-next-4.6/sys/dev/vt/vt_core.c:1465
#5  0xffffffff80a78f12 in cngrab () at
/usr/home/mmacy/drm-next-4.6/sys/kern/kern_cons.c:368
#6  0xffffffff80ae88c6 in vpanic (fmt=0xffffffff810f771b "%s",
ap=0xfffffe0461098e70)
    at /usr/home/mmacy/drm-next-4.6/sys/kern/kern_shutdown.c:745
#7  0xffffffff80ae87b3 in panic (fmt=<unavailable>) at
/usr/home/mmacy/drm-next-4.6/sys/kern/kern_shutdown.c:690
#8  0xffffffff80fc0361 in trap_fatal (frame=0xfffffe0461099170, eva=100)
    at /usr/home/mmacy/drm-next-4.6/sys/amd64/amd64/trap.c:841
#9  0xffffffff80fc0553 in trap_pfault (frame=0xfffffe0461099170, usermode=0)
    at /usr/home/mmacy/drm-next-4.6/sys/amd64/amd64/trap.c:691
#10 0xffffffff80fbfafc in trap (frame=0xfffffe0461099170) at
/usr/home/mmacy/drm-next-4.6/sys/amd64/amd64/trap.c:442
#11 <signal handler called>
#12 taskqueue_drain (queue=0x0, task=0xfffffe004fc17150) at
/usr/home/mmacy/drm-next-4.6/sys/kern/subr_taskqueue.c:554
#13 0xffffffff827823bb in ieee80211_draintask (ic=<optimized out>,
task=0xfffffe004fc17150)
    at /usr/home/mmacy/drm-next-4.6/sys/net80211/ieee80211_var.h:796
#14 iwm_detach_local (sc=<optimized out>, do_net80211=<optimized out>)
    at /usr/home/mmacy/drm-next-4.6/sys/modules/iwm/../../dev/iwm/if_iwm.c:6121
#15 0xffffffff80b21e8c in run_interrupt_driven_config_hooks ()
    at /usr/home/mmacy/drm-next-4.6/sys/kern/subr_autoconf.c:118
#16 0xffffffff80b21ccb in config_intrhook_establish (hook=<optimized out>)
    at /usr/home/mmacy/drm-next-4.6/sys/kern/subr_autoconf.c:182
#17 0xffffffff82781d03 in iwm_attach (dev=<optimized out>)
    at /usr/home/mmacy/drm-next-4.6/sys/modules/iwm/../../dev/iwm/if_iwm.c:5825
#18 0xffffffff80b26340 in DEVICE_ATTACH (dev=0xfffff8000744f700) at
./device_if.h:180
#19 device_attach (dev=0xfffff8000744f700) at
/usr/home/mmacy/drm-next-4.6/sys/kern/subr_bus.c:2900
#20 0xffffffff8071730d in pci_driver_added (dev=<optimized out>,
driver=<optimized out>)
    at /usr/home/mmacy/drm-next-4.6/sys/dev/pci/pci.c:4330
#21 0xffffffff80b23e9d in BUS_DRIVER_ADDED (_dev=0xfffff8000744f800,
_driver=0xffffffff82790c08 <iwm_pci_driver>)
    at ./bus_if.h:204
#22 devclass_driver_added (dc=<optimized out>, driver=<optimized out>)
    at /usr/home/mmacy/drm-next-4.6/sys/kern/subr_bus.c:1099
#23 0xffffffff80b23e02 in devclass_add_driver (dc=<optimized out>,
driver=<optimized out>, pass=<optimized out>,
    dcp=<optimized out>) at
/usr/home/mmacy/drm-next-4.6/sys/kern/subr_bus.c:1172
#24 0xffffffff80ac2300 in module_register_init (arg=<optimized out>)
    at /usr/home/mmacy/drm-next-4.6/sys/kern/kern_module.c:123
#25 0xffffffff80ab3f14 in linker_file_sysinit (lf=<optimized out>)
    at /usr/home/mmacy/drm-next-4.6/sys/kern/kern_linker.c:234
#26 linker_load_file (filename=<optimized out>, result=<optimized out>)
    at /usr/home/mmacy/drm-next-4.6/sys/kern/kern_linker.c:434
#27 linker_load_module (kldname=<optimized out>,
modname=0xfffff8000f0a8000 "if_iwm", parent=<optimized out>,
    verinfo=<optimized out>, lfpp=<optimized out>) at
/usr/home/mmacy/drm-next-4.6/sys/kern/kern_linker.c:2024
#28 0xffffffff80ab5d48 in kern_kldload (td=<optimized out>,
file=<optimized out>, fileid=0xfffffe0461099874)
    at /usr/home/mmacy/drm-next-4.6/sys/kern/kern_linker.c:1041
#29 0xffffffff80ab5eab in sys_kldload (td=0xfffff800966f1500,
uap=<optimized out>)
    at /usr/home/mmacy/drm-next-4.6/sys/kern/kern_linker.c:1067
#30 0xffffffff80fc0cc8 in syscallenter (td=<optimized out>, sa=<optimized out>)
    at /usr/home/mmacy/drm-next-4.6/sys/amd64/amd64/../../kern/subr_syscall.c:135
#31 amd64_syscall (td=<optimized out>, traced=0) at
/usr/home/mmacy/drm-next-4.6/sys/amd64/amd64/trap.c:942
#32 <signal handler called>
#33 0x000000080086d38a in ?? ()