undefined in result

[dy]

I invoke node build.js, where I have:

var stitch  = require('stitch');
var fs      = require('fs');

var package = stitch.createPackage({
    paths: [__dirname + '/src']
});

package.compile(function (err, source){
    fs.writeFile('dist/result.js', source, function (err) {
        if (err) throw err;
        console.log('Compiled result.js');
    })
})

But in result.js I see only undefined.