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