setting redirect to true redirects to undefined

[oupala]

I set metalsmith-aliases to redirect assets:

  .use(aliases({
    redirect: true
  }))

But when I browse the alised url, I get redirected to undefined instead of the real document url:

http://server.example.org/undefined

Am I using the plugin wrongly, or is there a problem while redirecting?

The README file is not documenting how to use the redirect feature si I had to look in the code. But I'm not sure that I'm using it the right way.