panic: bad mcs rate index 255 
cpuid = 0 
time = 1520001464
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe00005f92e0
vpanic() at vpanic+0x10d/frame 0xfffffe00005f9340
vpanic() at vpanic/frame 0xfffffe00005f93c0
r92c_tx_protection() at r92c_tw_protection+0x10e/frame 0xfffffe00005f9400
r92c_fill_tx_desc() at r92c_fill_tx_desc+0x240/frame 0xfffffe00005f9460
rtwn_tx_data() at rtwn_tx_data+0x1af/frame 0xfffffe00005f9500
rtwn_start() at rtwn_start+0xc7/frame 0xfffffe00005f9530
rtwn_transmit() at rtwn_transmit+0x0d/frame 0xfffffe00005f9560
ieee80211_parent_xmitpkt() at ieee80211_parent_xmitpkt+0x36/frame 0xfffffe00005f9590
ieee80211_vap_pkg_send_dest() at ieeee0211_vap_pkt_send_dest+0x29e/frame0xfffffe00005f95e0
ieee80211_vap_transmit at ieee88211_vap_transmit+0x2aa/frame 0xfffffe00005f9620
ether_output() at ether_output+0x6ef/frame 0xfffffe00005f96c0
ip_output() at ip_output+0x15fa/frame 0xfffffe00005f9800
udp_send() at udp_send+0xaaa/frame 0xfffffe00005f98d0
sosend_dgram() at sosend_dgram+0x395/frame 0xfffffe00005f9930
sosend() at sosend+0x6d/frame 0xfffffe00005f9970
kern_sendit() at kern_sendit+0x28c/frame 0xfffffe00005f9a20
sendit() at sendit+0x19e/frame 0xfffffe000005f9a70
sys_sendto() at sys_sendto+0x4d/frame 0xfffffe00005f9ac0
amd64_syscall() at amd64_syscall+0x79b/frame 0xfffffe00005f9bf0
fast_syscall_common() at fast_syscall_commond+0x101/frame 0x7fffffffdec0
KDB: enter: panic
[ thread pid 1493 tid 100135 ]
Stopped at  kdb_enter+0x3b: movq $0.kdb_why
db>