ifconfig: BRDGADD tun0: Invalid argument

[Boris Kochergin]

Ahoy. I have an 8.0-RELEASE/i386 machine (installed clean from the CD, 
so no kernel/world mismatches are possible) on which I am trying to add 
a tun device to a bridge:

# ifconfig tun0 create
# ifconfig bridge0 create
# ifconfig
...
tun0: flags=8010<POINTOPOINT,MULTICAST> metric 0 mtu 1500
bridge0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 
1500
        ether 5e:e7:af:1b:14:d3
        id 00:00:00:00:00:00 priority 32768 hellotime 2 fwddelay 15
        maxage 20 holdcnt 6 proto rstp maxaddr 100 timeout 1200
        root id 00:00:00:00:00:00 priority 32768 ifcost 0 port 0
# ifconfig bridge0 addm tun0
ifconfig: BRDGADD tun0: Invalid argument
# ifconfig tun0 promisc
# ifconfig bridge0 addm tun0
ifconfig: BRDGADD tun0: Invalid argument

I have looked at 
http://lists.freebsd.org/pipermail/freebsd-net/2007-December/016114.html, 
but none of the three possibilities appear to apply. Any clues?

-Boris