Rails 3.2 Precompile error

Every so often I come across an exception that doesn’t turn up good search results as a solution. This time it was:

$ rake assets:precompile --trace
...
NoMethodError: undefined method `empty?' for nil:NilClass
  (in <snip> /app/assets/stylesheets/application.css.scss)
<snip>/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sass-3.1.16/lib/sass/tree/root_node.rb:23:in `render'

The hint to the problem is actually in the above error, but without context would be difficult to spot (I didn’t see it with context)

The app in question is now a rails 3.2 app. However it didn’t start it’s life that way, it was slowly upgraded from 2.x, and had it’s own asset handling. The error occurred during a migration to the standard asset pipeline.

Sure enough I had gem 'sass-rails', '~> 3.2.0' in one area of the Gemfile and gem 'sass', '3.1.16' in another.