ipfw's "or-blocks" feature isn't working?

[Darren Pilgrim]

In my ruleset I have a rule with an "or-block" in it:

ipfw add pass tcp from \{ ${oip} or ${inet} \} to 192.168.100.1 80 keep-state

But when ipfw loads the rules, it returns "ipfw: hostname ''{'' unknown".  The
man page says the syntax is fine, so I'm stumped.  Can someone explain why this
isn't working?