cf push: pathname.rb - can't convert nil into String (TypeError)

When you updating the buildpack of your Ruby application it could be possible that you will see this error when pushing the application:

-----> Installing dependencies using 1.6.3
       Purging Cache. Changing stack from cedar to
       Fixing nokogiri install. Clearing bundler cache.
       See https://github.com/sparklemotion/nokogiri/issues/923.
/usr/lib/ruby/1.9.1/pathname.rb:308:in `initialize': can't convert nil into String (TypeError)
	from /usr/lib/ruby/1.9.1/pathname.rb:308:in `new'
	from /usr/lib/ruby/1.9.1/pathname.rb:308:in `+'
	from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/cache.rb:14:in `clear'
	from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/helpers/bundler_cache.rb:22:in `clear'
	from /tmp/buildpacks/heroku-buildpack-ruby/lib/language_pack/ruby.rb:830:in `block in purge_bundler_cache'
	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'

In this case just delete the application and push it again.