bourbon: Undefined variable: "$em-base"

I’m getting this error when assets:precompile runs…

The deploy has failed with an error: #<SSHKit::Runner::ExecuteError: Exception while executing on host cfs.staging: rake exit status: 1
rake stdout: Nothing written
rake stderr: /usr/local/rvm/rubies/ruby-2.1.2/bin/ruby /usr/local/rvm/gems/ruby-2.1.2@cfs-web/bin/rake assets:precompile:all RAILS_ENV=staging RAILS_GROUPS=assets
rake aborted!
Sass::SyntaxError: Undefined variable: "$em-base".
  (in /usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/bourbon-4.1.1/app/assets/stylesheets/functions/_modular-scale.scss)
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/bourbon-4.1.1/app/assets/stylesheets/functions/_modular-scale.scss:21
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/script/tree/variable.rb:49:in `_perform'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/script/tree/node.rb:50:in `perform'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/script/tree/funcall.rb:170:in `perform_arg'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/script/tree/funcall.rb:124:in `block in _perform'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/script/tree/funcall.rb:124:in `each'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/script/tree/funcall.rb:124:in `each_with_index'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/script/tree/funcall.rb:124:in `each'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/script/tree/funcall.rb:124:in `map'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/script/tree/funcall.rb:124:in `_perform'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/script/tree/node.rb:50:in `perform'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/perform.rb:468:in `visit_variable'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/base.rb:36:in `visit'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/perform.rb:158:in `block in visit'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/stack.rb:79:in `block in with_base'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/stack.rb:115:in `with_frame'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/stack.rb:79:in `with_base'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/perform.rb:158:in `visit'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/base.rb:52:in `block in visit_children'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/base.rb:52:in `map'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/base.rb:52:in `visit_children'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/perform.rb:167:in `block in visit_children'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/perform.rb:179:in `with_environment'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/perform.rb:166:in `visit_children'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/base.rb:36:in `block in visit'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/perform.rb:186:in `visit_root'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/base.rb:36:in `visit'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/perform.rb:157:in `visit'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/visitors/perform.rb:8:in `visit'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/root_node.rb:36:in `css_tree'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/tree/root_node.rb:20:in `render'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-3.4.10/lib/sass/engine.rb:268:in `render'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sass-rails-3.2.6/lib/sass/rails/template_handlers.rb:106:in `evaluate'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/tilt-1.4.1/lib/tilt/template.rb:103:in `render'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/context.rb:193:in `block in evaluate'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/context.rb:190:in `each'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/context.rb:190:in `evaluate'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/turbo-sprockets-rails3-0.3.14/lib/turbo-sprockets/sprockets_overrides/processed_asset.rb:16:in `initialize'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/turbo-sprockets-rails3-0.3.14/lib/turbo-sprockets/sprockets_overrides/base.rb:18:in `new'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/turbo-sprockets-rails3-0.3.14/lib/turbo-sprockets/sprockets_overrides/base.rb:18:in `block in build_asset'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/base.rb:270:in `circular_call_protection'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/turbo-sprockets-rails3-0.3.14/lib/turbo-sprockets/sprockets_overrides/base.rb:14:in `build_asset'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/index.rb:93:in `block in build_asset'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/caching.rb:19:in `cache_asset'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/index.rb:92:in `build_asset'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/base.rb:169:in `find_asset'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/turbo-sprockets-rails3-0.3.14/lib/turbo-sprockets/sprockets_overrides/index.rb:14:in `find_asset'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/turbo-sprockets-rails3-0.3.14/lib/turbo-sprockets/sprockets_overrides/bundled_asset.rb:12:in `initialize'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/turbo-sprockets-rails3-0.3.14/lib/turbo-sprockets/sprockets_overrides/base.rb:22:in `new'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/turbo-sprockets-rails3-0.3.14/lib/turbo-sprockets/sprockets_overrides/base.rb:22:in `build_asset'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/index.rb:93:in `block in build_asset'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/caching.rb:19:in `cache_asset'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/index.rb:92:in `build_asset'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/base.rb:169:in `find_asset'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/turbo-sprockets-rails3-0.3.14/lib/turbo-sprockets/sprockets_overrides/index.rb:14:in `find_asset'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/turbo-sprockets-rails3-0.3.14/lib/turbo-sprockets/sprockets_overrides/static_compiler.rb:41:in `block in compile'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/base.rb:219:in `block in each_logical_path'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/base.rb:206:in `block (2 levels) in each_file'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/base.rb:196:in `each'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/base.rb:196:in `each_entry'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/base.rb:204:in `block in each_file'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/base.rb:203:in `each'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/base.rb:203:in `each_file'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/sprockets-2.2.3/lib/sprockets/base.rb:217:in `each_logical_path'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/turbo-sprockets-rails3-0.3.14/lib/turbo-sprockets/sprockets_overrides/static_compiler.rb:29:in `compile'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/turbo-sprockets-rails3-0.3.14/lib/turbo-sprockets/tasks/assets.rake:108:in `internal_precompile'
/usr/local/rvm/gems/ruby-2.1.2@cfs-web/gems/turbo-sprockets-rails3-0.3.14/lib/turbo-sprockets/tasks/assets.rake:115:in `block (3 levels) in <top (required)>'
Tasks: TOP => assets:precompile:all
(See full trace by running task with --trace)
rake aborted!