Trouble with first Node app deployment through Heroku

Having followed the 12devs of Xmas tutorial to create my first Node.js app (http://12devsofxmas.co.uk/post/2012-12-28-day-3-realtime-collaborative-drawing-with-nodejs) I'm now attempting to follow Rob Dodson's tutorial for deploying it through Heroku (http://robdodson.me/blog/2012/06/04/deploying-your-first-node-dot-js-and-socket-dot-io-app-to-heroku/)

However, after committing to github, when I attempt to push to Heroku, I get the following error output:

-----> Installing dependencies with npm
       npm WARN package.json Draw@0.0.1 No README.md file found!
       npm http GET https://registry.npmjs.org/express
       npm http 200 https://registry.npmjs.org/express
       npm http GET https://registry.npmjs.org/express/-/express-2.5.11.tgz
       npm http 200 https://registry.npmjs.org/express/-/express-2.5.11.tgz
       npm ERR! Refusing to delete: /tmp/build_1t6d7hbd2psal/node_modules/.bin/express not in /tmp/build_1t6d7hbd2psal/node_modules/express
       File exists: /tmp/build_1t6d7hbd2psal/node_modules/.bin/express
       Move it away, and try again.

       npm ERR! System Linux 2.6.32-350-ec2
       npm ERR! command "/tmp/node-node-Fenl/bin/node" "/tmp/node-npm-6pyT/cli.js" "install" "--production"
       npm ERR! cwd /tmp/build_1t6d7hbd2psal
       npm ERR! node -v v0.6.20
       npm ERR! npm -v 1.1.65
       npm ERR! path /tmp/build_1t6d7hbd2psal/node_modules/.bin/express
       npm ERR! code EEXIST
       npm ERR!
       npm ERR! Additional logging details can be found in:
       npm ERR!     /tmp/build_1t6d7hbd2psal/npm-debug.log
       npm ERR! not ok code undefined
       npm ERR! not ok code 1
!     Failed to install --production dependencies with npm

!     Heroku push rejected, failed to compile Node.js app

To git@heroku.com:secure-dawn-2437.git

! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'git@heroku.com:secure-dawn-2437.git'

My package.json file is as follows:

{
    "name": "Draw",
    "version": "0.0.1",
    "private": true,
    "dependencies": {
      "express": "~2.x",
      "socket.io": "~0.9.x"
    },
    "engines": {
      "node": "0.6.x",
      "npm": "1.1.x"
    }
}

I'm sorry - I'm totally new to this sort of development, having mostly worked in Java while in school, but I'm anxious to learn. I assume the problem is somehow with express, but I'm not sure what it means by "move it away, and try again."