AWS Lambda copyObject "Process exited before completing request"

I am trying to copy files uploaded to an S3 bucket to create timestamped backups. (the 'live' file will overwritten periodically to maintain a perma-link)

AWScopyObject, however, does not seem to be executing. I am getting the following error

 {
       "errorMessage": "Process exited before completing request"
 }

The code that causes this error is as follows;

    console.log('Loading function');
var AWS = require('aws-sdk');
var s3 = new AWS.S3();

exports.handler = function(event, context) {
    var srcBucket = event.Records[0].s3.bucket.name;
    var srcKey    = event.Records[0].s3.object.key;
    var dstKey    = srcBucket+'/backup/'+ Date.now() + '-' +srcKey;

    console.log(srcKey);
    console.log(dstKey);

    var copyParams = {
        Bucket: srcBucket,
        CopySource : srcBucket + '/' + srcKey,
        Key: dstKey
    };

s3.AWScopyObject(
    copyParams,
    function (err, data) {
        if (err) {
            console.log("ERROR copyObject");
            console.log(err);
        }
        else {
            console.log('SUCCESS copyObject');
        }
        context.done();
    });

};

What could be causing lamda to exit before the AWSCopyObject callback? According to http://ift.tt/1z1C5E8 this error suggests context.done() is never hit.