Error output from ndp -an

I'm using OpenBSD 6.4 on a pcengines apu2 box as a router/firewall for a
CenturyLink DSL (pppoe) connection.

Today I set up rd6 for ipv6 for the first time, similar to what is
described here:
https://gist.github.com/afresh1/791343380b4410687d51fdd94f20bd42

Things are working well but one minor issue I notice is when I run "ndp
-an" to see ipv6 neighbor info I get an error printed to stderr from ndp:

[aaron@apu2] ~$ ndp -an
Neighbor Linklayer Address Netif Expire S
Flags
ndp: ioctl(SIOCGNBRINFO_IN6): Invalid argument
ndp: failed to get neighbor information

This is followed by normal ndp output lines so this seems like a very minor
bug, but is slightly noisy.

Below I am pasting:
1. ktrace output for the "ndp -an" command. Looks like
ioctl(SIOCGNBRINFO_IN6) is being called on a SOCK_DGRAM socket. ioctl
returns -1 with errno 22 Invalid Argument.
2. ifconfig output


1. ktrace output from "ndp -an" command:
9697 ndp CALL socket(AF_INET6,0x2<SOCK_DGRAM>,0)
9697 ndp RET socket 3
9697 ndp CALL
sysctl(4.17.0.0.6.0<net.route.0.0.6.0>,0,0x7f7ffffee808,0,0)
9697 ndp RET sysctl 0
9697 ndp CALL
sysctl(4.17.0.0.6.0<net.route.0.0.6.0>,0x133e91ee1a00,0x7f7ffffee808,0,0)
9697 ndp RET sysctl 0
9697 ndp CALL kbind(0x7f7ffffee7c8,24,0xd76384b171042686)
9697 ndp RET kbind 0
9697 ndp CALL ioctl(3,SIOCGNBRINFO_IN6,0x133c63e05100)
9697 ndp RET ioctl -1 errno 22 Invalid argument
9697 ndp CALL kbind(0x7f7ffffee7c8,24,0xd76384b171042686)
9697 ndp RET kbind 0
9697 ndp CALL write(2,0x7f7ffffee050,0x5)
9697 ndp GIO fd 2 wrote 5 bytes
"ndp: "
9697 ndp RET write 5
9697 ndp CALL write(2,0x7f7ffffee140,0x17)
9697 ndp GIO fd 2 wrote 23 bytes
"ioctl(SIOCGNBRINFO_IN6)"

2. ifconfig:
[aaron@apu2] ~$ ifconfig
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 32768
index 5 priority 0 llprio 3
groups: lo
inet6 ::1 prefixlen 128
inet6 fe80::1%lo0 prefixlen 64 scopeid 0x5
inet 127.0.0.1 netmask 0xff000000
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
lladdr 00:0d:b9:4b:17:10
index 1 priority 0 llprio 3
media: Ethernet autoselect (1000baseT full-duplex)
status: active
em1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
lladdr 00:0d:b9:4b:17:11
index 2 priority 0 llprio 3
media: Ethernet autoselect (1000baseT full-duplex,rxpause,txpause)
status: active
inet 192.168.1.1 netmask 0xffffff00 broadcast 192.168.1.255
inet6 fe80::813b:5c49:d64b:f9fc%em1 prefixlen 64 scopeid 0x2
inet6 2602:d8:a032:2200::1 prefixlen 64
em2: flags=8802<BROADCAST,SIMPLEX,MULTICAST> mtu 1500
lladdr 00:0d:b9:4b:17:12
index 3 priority 0 llprio 3
media: Ethernet autoselect (none)
status: no carrier
enc0: flags=0<>
index 4 priority 0 llprio 3
groups: enc
status: active
pppoe0: flags=8851<UP,POINTOPOINT,RUNNING,SIMPLEX,MULTICAST> mtu 1492
index 6 priority 0 llprio 3
dev: vlan201 state: session
sid: 0x25d PADI retries: 4 PADR retries: 0 time: 04:57:57
sppp: phase network authproto chap
groups: pppoe egress
status: active
inet 216.160.50.34 --> 207.109.2.20 netmask 0xffffffff
vlan201: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
lladdr 00:0d:b9:4b:17:10
index 7 priority 0 llprio 3
encap: vnetid 201 parent em0
groups: vlan
media: Ethernet autoselect (1000baseT full-duplex)
status: active
pflog0: flags=141<UP,RUNNING,PROMISC> mtu 33136
index 8 priority 0 llprio 3
groups: pflog
gif0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1472
index 9 priority 0 llprio 3
groups: gif egress
tunnel: inet 216.160.50.34 -> 205.171.2.64 ttl 64 nodf
inet6 fe80::e39e:d302:d26:902f%gif0 -> prefixlen 64 scopeid 0x9
inet6 2602:d8:a032:2200::1 -> prefixlen 24