nw-builder: winIco doesnt apply

Hi guys, I want to apply an icon to my release build but it is not integrated into the exe. Switched from nodebob to node-webkit-builder or to be exactly: grunt-node-webkit-builder. I have tried several images and several image types but nothing is changing. My config:

nodewebkit: {
            options: {
                build_dir: './dist',
                winIco: './app/app.ico',
                // specifiy what to build
                mac: false,
                win: true,
                linux32: false,
                linux64: false
            },
            src: './app/**/*'
        },

My folder structure:

package.json
Gruntfile.js
dist/
app/
 |-package.json
 |-index.html
 |-app.ico

I build on Win32. Did not help:

ie4uinit.exe -ClearIconCache