Problem with network interface

[Konrad Neuwirth <konrad%fimsch.net@localhost>]

Dear reader, 

I have a network problem on a system running NetBSD 5.1 that I do not fully 
understand. We have an interface wm2 that is configured as follows: 

wm2: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        
capabilities=7ff80<TSO4,IP4CSUM_Rx,IP4CSUM_Tx,TCP4CSUM_Rx,TCP4CSUM_Tx,UDP4CSUM_Rx,UDP4CSUM_Tx,TCP6CSUM_Rx,TCP6CSUM_Tx,UDP6CSUM_Rx,UDP6CSUM_Tx,TSO6>
        
enabled=3f80<TSO4,IP4CSUM_Rx,IP4CSUM_Tx,TCP4CSUM_Rx,TCP4CSUM_Tx,UDP4CSUM_Rx,UDP4CSUM_Tx>
        address: 00:15:17:b1:b7:ff
        media: Ethernet autoselect (100baseTX 
full-duplex,flowcontrol,rxpause,txpause)
        status: active
        inet 192.168.100.1 netmask 0xffffff00 broadcast 192.168.100.255
        inet6 fe80::215:17ff:feb1:b7ff%wm2 prefixlen 64 scopeid 0x3

Directly connected to it is a second machine, currently configured as 
192.168.100.2. We're trying to send pings back and forth. 
Ipmon tells us these things are even successful: 

Jul  1 10:56:26 pinie ipmon[24589]: 10:56:25.847016 wm2 @0:2 p 192.168.100.2 -> 
192.168.100.1 PR icmp len 20 60 icmp echo/0 IN
Jul  1 10:56:26 pinie ipmon[24589]: 10:56:25.847023 wm2 @0:2 p 192.168.100.1 -> 
192.168.100.2 PR icmp len 20 60 icmp echoreply/0 OUT

Alas, dmesg reports something else: 

arplookup: unable to enter address for 192.168.100.2@00:24:54:de:59:bd on wm2 
(host is not on local network)

Of course, this leads to no packets ever reaching the destination. 

What am I missing? Why can't the address be entered into the arp table? 

Thanks,
 Konrad