Emberjs: _instantiatedStack error using broccoli-funnel

I'm trying to copy my font-awesome fonts to my dist/fonts directory using this:

var fontFiles = funnel('bower_components/font-awesome/', {
    srcDir: 'fonts',
    destDir: 'fonts'
});
console.log(fontFiles);

module.exports = app.toTree(fontFiles);

return app.toTree();

Using the console.log I see this:

version: 1.13.1 { inputTree: 'bower_components/font-awesome/',
_includeFileCache: {}, _destinationPathCache: {}, srcDir: 'fonts', destDir: 'fonts', _instantiatedStack: 'Error\n at new Funnel (/home/.......

I've changed around the src directory with no effect.