It's pretty easy to reproduce. Create a Gemfile with this content:
gem "jammit", "0.6.5", git: "https://github.com/documentcloud/jammit.git"
Then try to bundle. It dies with an error.
It should be fixed with #19, I have just released new version including two fixes (#19+#20), could you please check with rubygems-bundler 1.0.1
I did check it and it does seem to work.