proxy DEFAULT realm bug-o-feature?

[Mike]

I placed the following into my preproxy_users file:

DEFAULT Realm != "NULL"
        NAS-IP-Address := `%{Realm}`

But instead of the expected result, I am observing:

        NAS-IP-Address := 255.255.255.255

in the proxied requests.  This seems hardly intuitive, but I can't
quite figure out if this is a bug or a feature.  If this is a feature,
how can I "save" the Realm from being clobbered by the word DEFAULT?
If this is a bug, a fix would certainly be nice...