ifconfig: BRDGADD lo1: invalid argument

[org.freebsd.security at io7m.com]

Hello.

I'm trying to create a bridge interface to isolate some jails on
private addresses. I'm on a near-pristine install of 10.3, updated to
10.3-p5 via freebsd-update.

The virtual interface to which the jails will be bound:

# ifconfig lo1 create

The bridge:

# ifconfig bridge create
bridge0
# ifconfig bridge0 addm em0
# ifconfig bridge0 addm lo1
ifconfig: BRDGADD lo1: Invalid argument

I can find one reference to this error message online from 2007:

  https://lists.freebsd.org/pipermail/freebsd-net/2007-December/016102.html

... but this doesn't appear to be the problem I'm having (it seems as
though it was caused by an out-of-sync kernel and world, which I
obviously don't have).

Any ideas how to get this working?

M