Heroku buildpack compile failure: App staging failed in the buildpack compile phase

Error description: App staging failed in the buildpack compile phase

If your deployment using the heroku ruby buildpack fails with an error like this:

-----> Downloaded app package (32K)
-----> Downloaded app buildpack cache (11M)
Cloning into '/tmp/buildpacks/heroku-buildpack-ruby'...
 !
 ! key not found
 !
/tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/base.rb:30:in `fetch': key not found (KeyError)
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/base.rb:30:in `block in initialize'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:18:in `block (2 levels) in instrument'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:17:in `block in instrument'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:16:in `instrument'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:40:in `yield_with_block_depth'
 from /usr/lib/ruby/1.9.1/benchmark.rb:295:in `realtime'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/base.rb:48:in `instrument'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/base.rb:28:in `initialize'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/ruby.rb:40:in `initialize'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack.rb:20:in `new'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack.rb:20:in `block in detect'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:18:in `block (2 levels) in instrument'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:40:in `yield_with_block_depth'
 from /usr/lib/ruby/1.9.1/benchmark.rb:295:in `realtime'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack.rb:13:in `detect'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:17:in `block in instrument'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:16:in `instrument'
 from /tmp/buildpacks/heroku-buildpack-ruby/bin/compile:12:in `block in <main>'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:35:in `call'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:35:in `block in trace'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:18:in `block (2 levels) in instrument'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:40:in `yield_with_block_depth'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:17:in `block in instrument'
 from /usr/lib/ruby/1.9.1/benchmark.rb:295:in `realtime'
 from /tmp/buildpacks/heroku-buildpack-ruby/bin/compile:11:in `<main>'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:16:in `instrument'
 from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/instrument.rb:35:in `trace'
Staging failed: Buildpack compilation step failed

FAILED
Server error, status code: 400, error code: 170004, message: App staging failed in the buildpack compile phase

Please set the STACK env variable for your application to cedar in your deployment manifest like this:

env:
  STACK: cedar

We recommend to set the STACK environment variable when using the ruby heroku buildpack for each of your deployments on anynines.com .

For more information on this environment variable please refer to the heroku buildpack documentation.