Excluding subdirectory from being generated

I wanted to exclude assets/sass from being included into the generated `_site`:

``` yaml
exclude:
  - assets/sass
  - README.md
  - config.rb
```

However it still includes `assets/sass`. I tried adding a trailing slash, putting it inside quotation marks but none works?

![Directioner forever](http://i.imgur.com/on9jTBX.png)
