Sinon stub and test AWS-SDK promise

//Class under test

var AWS = require('aws-sdk');
var s3Export = function() {
   var s3Client = new AWS.S3();

   this.getObject => (params) => {
     return s3Client.getObject(params).promise();
   }
};

module.exports.S3 = new s3Export();

// Test Class

describe('s3-wrapper-tests', function() {
  var app, aws, s3, getObjectData = {Bucket: 'test-bucket', Key: 'test-file'}, deleteObjectData,
  deleteObjectsDatas, putObject;

before(() => {
   s3 = sinon.stub({
     getObject: function(params) {
         getObjectData = params; 
         return Promise.resolve( {Body: ['a','b','c']} );
    });
    aws = sinon.stub(AWS, 'S3');
    aws.returns(s3);
    app = proxyquire(modulePath, {'aws-sdk': aws } );
});

after(() => {
    aws.restore(); 
});

beforeEach(() => {
    s3.getObject.reset();
    s3.getObject.callsArgWith(0, getObjectData);
});

it('test message', () => {
return app.S3.getObject(
  {Bucket: 'test-bucket', Key: 'test-file'}).then(() => {
        const args = s3.getObject.getCall(0).args[0];
        expect(args).to.not.be.null;
        });
    });
});

The question I have is on Sinon's callsWithArgs method and the promise's aws-sdk. My question, how can I test this call was made with the correct parameters and return a promise as expected. As it stands now the test fails with Exception:

TypeError: argument at index 1 is not a function: undefined