Simple Captcha Rails 3

Hi,

I'm trying to use Simple Captcha with Rails 3. (https://github.com/
galetahub/simple-captcha)
I've installed it, and followed the controller based usage. But when I
go to my form on my site, I just see a textfield with the label (I
typed here):

<%= show_simple_captcha(:label => "human authentication") %>
The captcha image isn't displayed. Just a red cross (the 'image not
found' red cross that is).

I have imagemagick installed.

Thx,
Mathew

Oh and this is the error I get in my terminal:

"TypeError (can't convert nil into Integer):"

+1