Signature does not match for AWS using Node.js SDK

I'm unable to get a simple AWS Node.js EC2 example to work. Here's my code:

var AWS = require('aws-sdk');
AWS.config.loadFromPath('./config.json');
new AWS.EC2().describeInstances(function(err, data) {
    if (err) {
        console.log(err);
    } else {
        console.log(data);
    }
});

The error I get when running it looks like this:

{ [SignatureDoesNotMatch: The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.]
  message: 'The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.',
  code: 'SignatureDoesNotMatch',
  time: Wed Sep 03 2014 16:29:37 GMT-0700 (PDT),
  statusCode: 403,
  retryable: false }

Why am I getting this error and how do I resolve it?

I'm using Node.js v0.10.31 on an Ubuntu 14.04.1 LTS 64-bit desktop.

Download a complete example here or here.

crossposted here: https://forums.aws.amazon.com/thread.jspa?threadID=160122