undefined method `args=' for [[]]:Sass::Tree::FunctionNode deploying to heroku with rails 3.1

I have an app that I am trying to deploy to Heroku Cedar stack with rails 3.1.0.rc5. Some blogs that I followed implementing the migration to cedar and asset pipeline:

• http://metaskills.net/2011/07/29/use-compass-sass-framework-files-with-the-rails-3.1.0.rc5-asset-pipeline/
• http://devcenter.heroku.com/articles/rails31_heroku_cedar
• http://railsapps.github.com/rails-heroku-tutorial.html

After a git push to heroku, I ran the assets:precompile task:

heroku run rake -t assets:precompile --app myapp

The js files are compiled fine, however Sass bombs compiling application.css.scss with error:

rake aborted!
undefined method `args=' for [[]]:Sass::Tree::FunctionNode
  (in /app/app/assets/stylesheets/application.css.开发者_如何学Goscss)

Full output and stacktrace here: https://gist.github.com/1122683

running bundle exec rake assets:precompile locally executes fine without errors.

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It seems to be some kind of incompatibility between Sass 3.1.6 and blueprint; I added

gem 'sass', '3.1.5'

to my gemfile and that seems to have cleared it up