ActionView::Template::Error in Unit Tests (Rails 3 + Devise)

So I’m resuming development and trying to create coverage for existing code before attempting actual TDD and I get this in my unit tests:

ActionView::Template::Error: Missing host to link to! Please provide the :host parameter, set default_url_options[:host], or set :only_path to true

These are unit tests! This is madness! Full error:

AdviceTest:
    ERROR test: Advice should belong to user.  (1.53s) 
          ActionView::Template::Error: Missing host to link to! Please provide the :host parameter, set default_url_options[:host], or set :only_path to true
          /home/denis/.rvm/gems/ruby-1.9.2-p290/gems/actionpack-3.1.0/lib/action_dispatch/http/url.rb:25:in `url_for'

The above problem creeps up when you’ve forgot to set mailer settings for Devise in your test environment.

open up config/environments/test.rb and a line to the likes of:

config.action_mailer.default_url_options = { :host => 'localhost:3000' }