Compass/Middleman image-url isn't working

I'm currently working with Middleman to build up a new page, but now the image-url helper doesn't seem to be working.

In my config.rb I have:

#Use relative URLs   
activate :relative_assets

set :css_dir, 'stylesheets' 
set :js_dir, 'javascripts'
set :images_dir, 'images'

In my applications.css.sass I have this:

body
  background: image_url("background.png")

The background image is in images/background but Middleman doesn't find/load that image. It just puts this out:

body {
  background: url("../images/background.png");
}

Does anyone know what I'm doing wrong?