Issue running express on Ubuntu 12.04

I had node installed on Ubuntu 11.10 and had express working. I then upgraded to 12.04 and suddenly I get the following stack trace:

node.js:201
         throw e; // process.nextTick error, or 'error' event on first tick
               ^
 ReferenceError: express is not defined
     at Object.<anonymous> (/home/cyrus/server.js:2:11)
     at Module._compile (module.js:441:26)
     at Object..js (module.js:459:10)
     at Module.load (module.js:348:31)
     at Function._load (module.js:308:12)
     at Array.0 (module.js:479:10)
     at EventEmitter._tickCallback (node.js:192:40)
 

I removed express using npm and then cleared my cache and ran the following:

npm install express;
 

Still I get the same stack trace indicated express is not defined, although npm ls shows that I have express 2.5.9 installed with connect version 1.8.7.

My node script is as follows:

var app = require('express').createServer();
 app.get('/', function(req,res){
 
 });
 

It worked I upgraded to Ubuntu 12.04.

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