captcha validation shuts webrick server down

i tried setting up the captcha validation plugin but anytime i go to a
page that has the image on it, the web server completely shuts down
and i get this:

./script/../config/../vendor/plugins/validates_captcha/lib/
captcha_image_challenge.rb:93: [BUG] Bus Error
ruby 1.8.4 (2005-12-24) [i686-darwin8.8.2]

Abort trap

i don't see anything in the log files so i'm not sure where to start
with this one. i'm developing on an intel macbook pro, and have
imagemagick 6.3.0 if that helps...

thanks in advance,
josh