RoR: annotate «undefined method `namespace' for main:Object»

If you get error like «undefined method `namespace' for main:Object» when you are trying to start bundle annotate,
just add

:git => 'git://github.com/jeremyolliver/annotate_models.git', :branch => 'rake_compatibility'
after gem 'annotate', into your Gemfile.

bundle install
bundle exec annotate