nil in assets path name

[NV]

I put the following code to my default.html:

{% css_asset_tag global %}
- /_assets/a.css
- /_assets/b.css
{% endcss_asset_tag %}

When I run jekyll build I’m getting

<link href='/nil/global-554d1da02802a6dcb04cd6d9b1e1b301.css' rel='stylesheet' type='text/css' />

Which results in not found, since I keep my files in "assets" directory, not "nil".

Am I doing something wrong?

[NV]

I copy-pasted the following snipped:

asset_pipeline:
  bundle: true            # Default = true
  compress: true          # Default = true
  output_path: assets     # Default = assets
  display_path: nil       # Default = nil
  gzip: false             # Default = false

to my _config.yml. display_path: nil caused the trouble. I removed this line and now it works correctly.