heroku - ative_admin フェッチ後に Heroku へのプッシュがハングする

Question

このHerokuへのプッシュが機能しない理由がわかりません。何か案は？

Fetching git://github.com/gregbell/active_admin.git
   fatal: Could not parse object '54fb71c5415f7b1a7aa0cdde680ee882fa3b2d6f'.
   Git error: command `git reset --hard 54fb71c5415f7b1a7aa0cdde680ee882fa3b2d6f`
   in directory
   /tmp/build_1660sx0l4npgk/vendor/bundle/ruby/2.0.0/bundler/gems/active_admin-54fb71c5415f
   has failed.
   If this error persists you could try removing the cache directory
   '/tmp/build_1660sx0l4npgk/vendor/bundle/ruby/2.0.0/cache/bundler/git/active_admin-d67faab65e9b74efbc8efb4a777a851e9f78b2ca'
   Bundler Output: Fetching gem metadata from https://rubygems.org/..........
   Fetching gem metadata from https://rubygems.org/..
   Fetching git://github.com/justinfrench/formtastic.git
   Fetching git://github.com/josevalim/inherited_resources.git
   Fetching git://github.com/ernie/ransack.git
   Fetching git://github.com/gregbell/active_admin.git
   fatal: Could not parse object '54fb71c5415f7b1a7aa0cdde680ee882fa3b2d6f'.
   Git error: command `git reset --hard 54fb71c5415f7b1a7aa0cdde680ee882fa3b2d6f`
   in directory
   /tmp/build_1660sx0l4npgk/vendor/bundle/ruby/2.0.0/bundler/gems/active_admin-54fb71c5415f
   has failed.
   If this error persists you could try removing the cache directory
   '/tmp/build_1660sx0l4npgk/vendor/bundle/ruby/2.0.0/cache/bundler/git/active_admin-d67faab65e9b74efbc8efb4a777a851e9f78b2ca'

! ! Bundler 経由で gem をインストールできませんでした。!

! プッシュが拒否され、Ruby/Rails アプリのコンパイルに失敗しました