Rack error (not compiling on the fly) serving assets in development

Question

Serving assets has started creating this error today:

!! Rack application returned nil body. Probably you wanted it to be an empty string?
!! Unexpected error while processing request: undefined method `each' for nil:NilClass

I am using coffeescript and SCSS (application.js.coffee and application.css.scss)

From Gemfile:

group :assets do
  gem 'sass-rails',   '~> 3.2.3'
  gem 'compass-rails'
  gem 'coffee-rails', '~> 3.2.1'
  gem 'marionette-rails'
  gem 'uglifier', '>= 1.0.3'
end

Using ruby 1.9.3-p392 and 1.9.3-p125 causes this problem. Google results for this error are near non-existant.

Answer 1

Will's answer above fixed it for me in one case, however I reached this issue again in another project and have found a similarity between the two.

Caching in development is causing this issue, in development.rb simply add/update the following line and restart the application:

config.action_controller.perform_caching = false

Answer 2

I was getting this same issue for a couple of days. I believe that the issue was caused by a conflict between the latest version of rails and older gems. Updating your gems should solve the issue (it did for me).