为什么这个RMagick调用会产生分割错误?

I've been banging my head against the wall for the better part of an hour trying to figure out what's going wrong here, and I'm sure (or rather hoping) it's something fairly obvious that I'm overlooking. I'm using Ruby 1.9.1, Sinatra 1.0, and RMagick 2.13.1. ImageMagick and RMagick are correctly installed and functional—I've successfully manipulated and saved images from irb.I've been banging my head against the wall for