webpack es5 commonjs module results in empty exports object

I am trying to call a small library build with webpack2. It is configured to result in a es5, commonjs bundle. I am using nashorn-commonjs-module to import it by evaluating module = require('importModule');.
importModule contains a default export to a function served as the API, which I want to call.

However, module is evaluate to an empty object. There are no exports, nothing... any idea? Did I get something wrong?