undefined is not a function after refactor

Question

I put my code into different modules:

.....
var util = require('util');
var s3 = new AWS.S3();

exports.perform_flatten = function(event, context) {
     .......
     CONTEXT = context;

   s3_helper.get_object(SRC_BUCKET, SRC_KEY, this.flatten);
};

this.flatten is a callback to a module in the same file

exports.flatten = function(data){
    .......

 s3.putObject({Bucket: dst_bucket, Key: dst_key, Body: buffer}, function(err, data) {
    if (err){
      console.log(err, err.stack); // an error occurred
    }else{
      console.log(data);           // successful response
      CONTEXT.done();
    }
  });
}

The code runs perfectly until it comes to this line CONTEXT.done();.

Then I get the error: undefined is not a function

I do not understand why? Especially because I believe I declared CONTEXT as global right?

I have to call context.done() in order to execute a Lambda function on Amazon. http://docs.aws.amazon.com/lambda/latest/dg/programming-model.html THANKS