PDFKit and NodeJs creating damaged pdf documents

Using pdfkit with nodeJs the following code

        var expect = require('chai').expect;
        var PDFDocument = require('pdfkit');
        var fs = require('fs');


        doc = new PDFDocument();
        doc.pipe(fs.createWriteStream('output.pdf'));
        doc.fontSize(15);
        doc.text('Generate PDF!');

        doc.end();
        done();

Generates damaged documents ... why ???