MentosError (Failed to get header.)

I checked issue #45 but it doesn't seem to be the same case. I'm running my Rails app in production using Passenger and Nginx and I'm trying to highlight a piece of Ruby code. When I submit the code, I get a 500 error from my app. Looking at the Nginx logs, I see:

App 17862 stdout: Completed 500 Internal Server Error in 13ms
App 17862 stdout: 
App 17862 stdout: MentosError (Failed to get header.):
App 17862 stdout:   config/initializers/markdown_renderer.rb:3:in `block_code'
App 17862 stdout:   app/models/article.rb:16:in `render'
App 17862 stdout:   app/models/article.rb:16:in `cache_makdown'
App 17862 stdout:   app/controllers/articles_controller.rb:15:in `create'

I have checked all the python executables available on my Ubuntu server. They're all Python 2.7.6, which I installed from source a few hours ago.

$ which python
/usr/local/bin/python
$ which python2
/usr/local/bin/python2
$ python --version
Python 2.7.6
$python2 --version
Python 2.7.6

There's also another weird issue, where if I try using pygments from a ruby console, everything works fine:

$ irb
irb(main):001:0> require "pygments"
=> true
irb(main):002:0> Pygments.highlight("1 + 1", lexer: "ruby")
=> "<div class=\"highlight\"><pre><span class=\"mi\">1</span> <span class=\"o\">+</span> <span class=\"mi\">1</span>\n</pre></div>"

And even from the Rails console of the actual application:

$ RAILS_ENV=production bundle exec rails c
Loading production environment (Rails 4.0.0)
irb(main):001:0> Pygments.highlight("1 + 1", lexer: "ruby")
=> "<div class=\"highlight\"><pre><span class=\"mi\">1</span> <span class=\"o\">+</span> <span class=\"mi\">1</span>\n</pre></div>"

I am running Ruby 2.1.0, I have Ubuntu and Python installed from source. Does anyone know what could be the cause?

pygments / pygments.rb

MentosError (Failed to get header.) #105