How do I avoid moving read mail to ~/mbox?

[jmo]

I want to keep my read mail right where it is (I delete or refile
it to get rid of it).

I'm not fond of being asked each time I exit mutt if I would like
to move my mail off to ~/mbox.

I figured putting "mbox-hook $spool $spool" in my .muttrc might do
the trick, but it just causes a core dump on start up (at least
with Mutt 1.0.1i).

Is there a way to get mutt not to prompt me about moving read
messages when I exit?