IPython notebook that breaks python-markdown's pymdpreprocessor.py

fail.ipynb

{
 "metadata": {
  "name": "",
  "signature": "sha256:fae0b7120ebcb8202ca9dd96c97ae0ca4de84d9a4c4c18394365850d4809793a"
 },
 "nbformat": 3,
 "nbformat_minor": 0,
 "worksheets": [
  {
   "cells": [
    {
     "cell_type": "markdown",
     "metadata": {
      "variables": {
       "foo": {}
      }
     },
     "source": [
      "this is a nonexistent variable: {{foo}}"
     ]
    }
   ],
   "metadata": {}
  }
 ]
}