Mysterious Application Error on Heroku

I converted my app to rails 4. My test suite is green and it runs great
locally. I then deploy to Heroku and it crashed with no information at
It just says "Application Error. An error occurred in the application
and your page could not be served"

Heroku logs don't give any info either:

Send a proper trace from development environment, if it won't boot up
then the problem is either in a gem, gemfile or config/*