heroku precompiling asserts failed: Sass::SyntaxError: Undefined variable
<p>I develop an app based on M. Hartls tutorial. When I try to push my sample-app on heroku, the precomiling of the assets fails with the error:</p>

<blockquote>
  <p>Sass::SyntaxError: Undefined variable: "$gray-lighter"</p>
</blockquote>

<p>In none of my files I mention a "gray-lighter". 
My application.css.scss contains only: </p>

<pre><code>/*
 *= require_tree .
 *= require_self
 */
@import "bootstrap-sprockets";
@import "bootstrap";
</code></pre>

<p>Has anyone an idea how I could solve this problem?
Besides the error code - do you need more information?</p>

<pre><code>$ git push heroku master -f
Counting objects: 168, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (154/154), done.
Writing objects: 100% (168/168), 40.30 KiB | 0 bytes/s, done.
Total 168 (delta 32), reused 0 (delta 0)
remote: Compressing source files... done.
remote: Building source:
remote:
remote: -----&gt; Ruby app detected
remote: -----&gt; Compiling Ruby/Rails
remote: -----&gt; Using Ruby version: ruby-2.2.4
remote: ###### WARNING:
remote:        Removing `Gemfile.lock` because it was generated on Windows.
remote:        Bundler will do a full resolve so native gems are handled properly.
remote:        This may result in unexpected gem versions being used in your app.
remote:        In rare occasions Bundler may not be able to resolve your dependencies at all.
remote:        https://devcenter.heroku.com/articles/bundler-windows-gemfile
remote:
remote: -----&gt; Installing dependencies using bundler 1.9.7
remote:        Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin -j4
remote:        Fetching gem metadata from https://rubygems.org/...........
remote:        Fetching version metadata from https://rubygems.org/...
remote:        Fetching dependency metadata from https://rubygems.org/..
remote:        Resolving dependencies....
remote:        Installing i18n 0.7.0
remote:        Installing rake 10.5.0
remote:        Installing minitest 5.8.4
remote:        Installing thread_safe 0.3.5
remote:        Installing builder 3.2.2
remote:        Installing erubis 2.7.0
remote:        Installing mini_portile2 2.0.0
remote:        Installing rack 1.6.4
remote:        Installing json 1.8.3
remote:        Installing mime-types 2.99
remote:        Installing arel 6.0.3
remote:        Installing execjs 2.6.0
remote:        Using bundler 1.9.7
remote:        Installing coffee-script-source 1.10.0
remote:        Installing sass 3.4.21
remote:        Installing thor 0.19.1
remote:        Installing concurrent-ruby 1.0.0
remote:        Installing multi_json 1.11.2
remote:        Installing bcrypt 3.1.10
remote:        Installing libv8 3.16.14.13
remote:        Installing rails_serve_static_assets 0.0.5
remote:        Installing rails_stdout_logging 0.0.4
remote:        Installing ref 2.0.0
remote:        Installing tilt 2.0.2
remote:        Installing tzinfo 1.2.2
remote:        Installing rack-test 0.6.3
remote:        Installing pg 0.17.1
remote:        Installing mail 2.6.3
remote:        Installing autoprefixer-rails 6.3.1
remote:        Installing puma 2.11.1
remote:        Installing uglifier 2.7.2
remote:        Installing coffee-script 2.4.1
remote:        Installing bootstrap-sass 3.2.0.2
remote:        Installing sprockets 3.5.2
remote:        Installing rails_12factor 0.0.2
remote:        Installing activesupport 4.2.5
remote:        Installing rails-deprecated_sanitizer 1.0.3
remote:        Installing globalid 0.3.6
remote:        Installing activemodel 4.2.5
remote:        Installing jbuilder 2.4.0
remote:        Installing activejob 4.2.5
remote:        Installing activerecord 4.2.5
remote:        Installing nokogiri 1.6.7.2
remote:        Installing rails-dom-testing 1.0.7
remote:        Installing loofah 2.0.3
remote:        Installing rails-html-sanitizer 1.0.3
remote:        Installing actionview 4.2.5
remote:        Installing actionpack 4.2.5
remote:        Installing actionmailer 4.2.5
remote:        Installing railties 4.2.5
remote:        Installing coffee-rails 4.1.1
remote:        Installing sprockets-rails 3.0.1
remote:        Installing jquery-rails 4.1.0
remote:        Installing turbolinks 2.5.3
remote:        Installing rails 4.2.5
remote:        Installing sass-rails 5.0.4
remote:        Installing therubyracer 0.12.2
remote:        Bundle complete! 23 Gemfile dependencies, 57 gems now installed.
remote:        Gems in the groups development and test were not installed.
remote:        Bundled gems are installed into ./vendor/bundle.
remote:        Bundle completed (33.06s)
remote:        Cleaning up the bundler cache.
remote: -----&gt; Preparing app for Rails asset pipeline
remote:        Running: rake assets:precompile
remote:        I, [2016-02-05T18:12:00.582969 #1174]  INFO -- : Writing /tmp/build_f90401fc14a7fa141740de8ac82224af/public/assets/application-5caa7ce0a1e3d41955b9f91ccab3418d4f098eee350d3d8dcd9698c5fe32ca66.js
remote:        I, [2016-02-05T18:12:00.583759 #1174]  INFO -- : Writing /tmp/build_f90401fc14a7fa141740de8ac82224af/public/assets/application-5caa7ce0a1e3d41955b9f91ccab3418d4f098eee350d3d8dcd9698c5fe32ca66.js.gz
remote:        rake aborted!
remote:        Sass::SyntaxError: Undefined variable: "$gray-lighter".
remote:        (sass):73
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/script/tree/variable.rb:49:in `_perform'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/script/tree/node.rb:50:in `perform'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/script/tree/list_literal.rb:63:in `block in _perform'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/script/tree/list_literal.rb:63:in `map'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/script/tree/list_literal.rb:63:in `_perform'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/script/tree/node.rb:50:in `perform'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:395:in `visit_prop'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/base.rb:36:in `visit'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:158:in `block in visit'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/stack.rb:79:in `block in with_base'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/stack.rb:115:in `with_frame'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/stack.rb:79:in `with_base'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:158:in `visit'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:431:in `block (2 levels) in visit_rule'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:431:in `map'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:431:in `block in visit_rule'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:179:in `with_environment'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:429:in `visit_rule'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/base.rb:36:in `visit'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:158:in `block in visit'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/stack.rb:79:in `block in with_base'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/stack.rb:115:in `with_frame'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/stack.rb:79:in `with_base'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:158:in `visit'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:431:in `block (2 levels) in visit_rule'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:431:in `map'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:431:in `block in visit_rule'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:179:in `with_environment'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:429:in `visit_rule'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/base.rb:36:in `visit'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:158:in `block in visit'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/stack.rb:79:in `block in with_base'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/stack.rb:115:in `with_frame'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/stack.rb:79:in `with_base'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:158:in `visit'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/base.rb:52:in `block in visit_children'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/base.rb:52:in `map'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/base.rb:52:in `visit_children'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:167:in `block in visit_children'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:179:in `with_environment'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:166:in `visit_children'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/base.rb:36:in `block in visit'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:186:in `visit_root'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/base.rb:36:in `visit'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:157:in `visit'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/visitors/perform.rb:8:in `visit'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/root_node.rb:36:in `css_tree'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/tree/root_node.rb:20:in `render'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sass-3.4.21/lib/sass/engine.rb:278:in `render'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/sass_compressor.rb:48:in `call'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/sass_compressor.rb:28:in `call'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/processor_utils.rb:75:in `call_processor'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/processor_utils.rb:57:in `block in call_processors'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/processor_utils.rb:56:in `reverse_each'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/processor_utils.rb:56:in `call_processors'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/loader.rb:134:in `load_from_unloaded'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/loader.rb:60:in `block in load'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/loader.rb:318:in `fetch_asset_from_dependency_cache'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/loader.rb:44:in `load'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/cached_environment.rb:20:in `block in initialize'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/cached_environment.rb:47:in `yield'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/cached_environment.rb:47:in `load'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/base.rb:66:in `find_asset'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/base.rb:73:in `find_all_linked_assets'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/manifest.rb:142:in `block in find'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/legacy.rb:114:in `block (2 levels) in logical_paths'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/path_utils.rb:225:in `block in stat_tree'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/path_utils.rb:209:in `block in stat_directory'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/path_utils.rb:206:in `each'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/path_utils.rb:206:in `stat_directory'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/path_utils.rb:224:in `stat_tree'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/legacy.rb:105:in `each'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/legacy.rb:105:in `block in logical_paths'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/legacy.rb:104:in `each'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/legacy.rb:104:in `logical_paths'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/manifest.rb:140:in `find'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/sprockets/manifest.rb:168:in `compile'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-rails-3.0.1/lib/sprockets/rails/task.rb:68:in `block (3 levels) in define'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-3.5.2/lib/rake/sprocketstask.rb:147:in `with_logger'
remote:        /tmp/build_f90401fc14a7fa141740de8ac82224af/vendor/bundle/ruby/2.2.0/gems/sprockets-rails-3.0.1/lib/sprockets/rails/task.rb:67:in `block (2 levels) in define'
remote:        Tasks: TOP =&gt; assets:precompile
remote:        (See full trace by running task with --trace)
remote:  !
remote:  !     Precompiling assets failed.
remote:  !
remote:
remote:  !     Push rejected, failed to compile Ruby app
remote:
remote: Verifying deploy...
remote:
remote: !       Push rejected to myapp-30998.
remote:
To https://git.heroku.com/myapp-30998.git
 ! [remote rejected] master -&gt; master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/myapp-30998.git'
</code></pre>