Hieraki help

I need to set up a Wiki for a project that I'm working on, and HIeraki
seems to be the best rails-based option for my needs. I've got it
installed & with the help of some notes on the net, it's mostly
working. Unfortunately there are a few big bugs still, including one
when I try to create a new page. I'm new enough at Rails that my
debugging skills are seriously lacking. Can anyone offer any
suggestions? The relevant code is below the error message.

Any help is appreciated!