verrevpath -- ipfw: unknown argument ``not''

[Mark Edwards]

I am trying to implement the verrevpath suggestion in the ipfw man  
page, as follows:

>      The verrevpath option could be used to do automated anti- 
> spoofing by
>      adding the following to the top of a ruleset:
>
>            ipfw add deny ip from any to any not verrevpath in

However, when I try to add the rule, I get an error:

> lilbuddy:~ paimin$ ipfw add deny ip from any to any not verrevpath in
> ipfw: unknown argument ``not''

Can someone tell what is causing this syntax to fail?  Thanks!

--
Mark Edwards