Nodejs generating pdf to browser using pdfkit

I'm trying to display a PDF on my browser using Webtask.io and keep getting the following error:

{
  "code": 500,
  "error": "Script generated an unhandled asynchronous exception.",
  "details": "TypeError: dest.write is not a function",
  "name": "TypeError",
  "message": "dest.write is not a function",
  "stack": "TypeError: dest.write is not a function\n    at PDFDocument.ondata (_stream_readable.js:536:20)\n    at emitOne (events.js:77:13)\n    at PDFDocument.emit (events.js:169:7)\n    at PDFDocument.Readable.read (_stream_readable.js:368:10)\n    at flow (_stream_readable.js:751:26)\n    at resume_ (_stream_readable.js:731:3)\n    at nextTickCallbackWith2Args (node.js:442:9)\n    at process._tickDomainCallback (node.js:397:17)"
}

Does anyone know why this would be happening? Here is my code.

var fs = require('fs');
var PDFDocument = require('pdfkit@0.7.2');
var blobStream = require('blob-stream');
var ace = require('braces@1.8.0');

module.exports = 
  function (req,res,next) {

  doc = new PDFDocument(); 
  doc.pipe(res);

  doc.moveTo(300, 75)
   .lineTo(373, 301)
   .lineTo(181, 161)
   .lineTo(419, 161)
   .lineTo(227, 301)
   .fill('red', 'even-odd');  

  var loremIpsum = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Etiam in...';  

  doc.y = 320;
  doc.fillColor('black')
  doc.text(loremIpsum, {
     paragraphGap: 10,
     indent: 20,
     align: 'justify',
     columns: 2
  });  

  doc.end();

}

via bryan