undefined method `environment’ for nil:NilClass when importing Bootstrap into rails

I also met that exact same error after I upgraded rails from 4.0.3 to 4.0.4.

To solve solve this problem, only add

gem "sass-rails", "~> 4.0.2"

to your Gemfile and then

bundle install

Finally restart rails server

Then enjoy your programming in rails 4.0.4