NoMethodError in Users#index [], undefined method `[]' for nil:NilClass

I'm using Ruby version 1.9.3 and Rails 3.2.16 running on Ubuntu. I don't know what alter or update to fix this.

NoMethodError in Users#index

Showing /home/alex/Documents/curso_rails/demo_app/app/views/layouts/application.html.erb where line #5 raised:

undefined method '[]' for nil:NilClass

Extracted source (around line #5):

2: 
3: 
4:   DemoApp
5:   <%= stylesheet_link_tag "application", :media => "all" %>
6:   <%= javascript_include_tag "application" %>
7:   <%= csrf_meta_tags %>
8: