Heroku deploy failure on Cedar with 'include Rake::DSL' in Rakefile

gistfile1.txt

[~/sendgrid-example] git push heroku master
Counting objects: 5, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 367 bytes, done.
Total 3 (delta 2), reused 0 (delta 0)

-----> Heroku receiving push
-----> Rails app detected
-----> Installing dependencies using Bundler version 1.1.pre.1
       Checking for unresolved dependencies.
       All dependencies are satisfied.
/mnt/slug-compiler/language_packs/ruby/lib/language_pack/ruby.rb:129:in `rake_task_defined?': undefined method `detect' for nil:NilClass (NoMethodError)
	from /mnt/slug-compiler/language_packs/ruby/lib/language_pack/ruby.rb:109:in `run_compile_hook'
	from /mnt/slug-compiler/language_packs/ruby/lib/language_pack/ruby.rb:36:in `compile'
	from /mnt/slug-compiler/language_packs/ruby/lib/language_pack/rails2.rb:39:in `compile'
	from /mnt/slug-compiler/language_packs/ruby/lib/language_pack/rails3.rb:28:in `compile'
	from /mnt/slug-compiler/lib/../language_packs/ruby/bin/compile:7:in `<main>'
 !     Heroku push rejected, failed to compile Rails app

To git@heroku.com:sendgrid-example.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'git@heroku.com:sendgrid-example.git'

I came across this via Google... are you bundling Rake? If so, don't.

We'll work to improve this error.

I wasn't bundling rake initially, but I tried it just in case. You get the error either way.