aws-sdk-js: NotImplemented - A header you provided implies functionality that is not implemented

I am consistently receiving a NotImplemented error when trying to write a simple text file to S3 using the sdk.

var AWS = require( 'aws-sdk' );

AWS.config.update( { 
  accessKeyId : 'ACCESS_KEY_ID', 
  secretAccessKey : 'SECRET_ACCESS_KEY' 
});

var s3 = new AWS.S3({ endpoint : 'https://s3.amazonaws.com' });

s3.client.putObject({
  ACL : 'private',
  Body : 'some string',
  Bucket : 'my-bucket',
  Key : 'some/path/here/stuff.txt',
  ServerSideEncryption : 'AES256'
}, function ( error, response ) {
 // error listed below
});

results in the following error:

{
  "code":"NotImplemented",
  "message":"A header you provided implies functionality that is not implemented",
  "statusCode":501,
  "retryable":true
}

which, in itself seems weird considering it is NotImplemented but is retryable.