Rails 4 app on Heroku is 500ing, but no error message in logs

I've made a new Rails 4 beta app and deployed it to Heroku (using Ruby 2.0.0, and Unicorn via a Procfile). It runs fine locally with `foreman start`, but when I visit the homepage of the deployed app, I just get the standard Heroku 500 error: ``` We're sorry, but something went wrong. If you are the application owner check the logs for more information. ``` However, in the logs, there are no errors. I've no idea what is causing the 500. Here is the output from my last `git push heroku master`: ``` Counting objects: 6, done. Delta compression using up to 8 threads. Compressing objects: 100% (4/4), done. Writing objects: 100% (4/4), 656 bytes, done. Total 4 (delta 2), reused 0 (delta 0) -----> Ruby/Rails app detected -----> Using Ruby version: ruby-2.0.0 -----> Installing dependencies using Bundler version 1.3.2 Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin --deployment Using rake (10.0.4) Using i18n (0.6.4) Using minitest (4.7.3) Using multi_json (1.7.2) Using atomic (1.1.8) Using thread_safe (0.1.0) Using tzinfo (0.3.37) Using activesupport (4.0.0.beta1) Using builder (3.1.4) Using erubis (2.7.0) Using rack (1.5.2) Using rack-test (0.6.2) Using actionpack (4.0.0.beta1) Using mime-types (1.23) Using polyglot (0.3.3) Using treetop (1.4.12) Using mail (2.5.3) Using actionmailer (4.0.0.beta1) Using activemodel (4.0.0.beta1) Using activerecord-deprecated_finders (0.0.3) Using arel (4.0.0) Using activerecord (4.0.0.beta1) Using coffee-script-source (1.6.2) Using execjs (1.4.0) Using coffee-script (2.2.0) Using json (1.7.7) Using rdoc (3.12.2) Using thor (0.18.1) Using railties (4.0.0.beta1) Using coffee-rails (4.0.0) Using hike (1.2.2) Using jbuilder (1.0.2) Using jquery-rails (2.2.1) Using kgio (2.8.0) Using mysql2 (0.3.11) Using bundler (1.3.2) Using tilt (1.3.7) Using sprockets (2.9.3) Using sprockets-rails (2.0.0.rc4) Using rails (4.0.0.beta1) Using raindrops (0.11.0) Using sass (3.2.8) Using sass-rails (4.0.0.rc1) Using turbolinks (1.1.1) Using uglifier (2.0.1) Using unicorn (4.6.2) Your bundle is complete! It was installed into ./vendor/bundle Cleaning up the bundler cache. -----> Preparing app for Rails asset pipeline Running: rake assets:precompile I, [2013-04-23T22:28:44.559735 #871] INFO -- : Writing /tmp/build_33m76p0rij8a5/public/assets/rails-d8fb6a2617998f32edf5edb354fc5326.png I, [2013-04-23T22:28:49.911890 #871] INFO -- : Writing /tmp/build_33m76p0rij8a5/public/assets/application-d4a508061cd56ce9e544945495285e0c.js I, [2013-04-23T22:28:50.120141 #871] INFO -- : Writing /tmp/build_33m76p0rij8a5/public/assets/application-a3b89b66a8471f3e75abb239e3b35c7d.css Asset precompilation completed (8.30s) -----> Rails plugin injection -----> Discovering process types Procfile declares types -> web Default types for Ruby/Rails -> console, rake, worker -----> Compiled slug size: 35.0MB -----> Launching... done, v21 http://blofs.herokuapp.com deployed to Heroku To git@heroku.com:blofs.git 32e51a2..37d989c master -> master ``` And here are the logs from that push: ``` 2013-04-23T22:28:06.650049+00:00 heroku[slugc]: Slug compilation started 2013-04-23T22:29:11.194613+00:00 heroku[api]: Release v21 created by callum.locke@gmail.com 2013-04-23T22:29:11.365908+00:00 heroku[api]: Deploy 37d989c by callum.locke@gmail.com 2013-04-23T22:29:11.441066+00:00 heroku[web.1]: State changed from up to starting 2013-04-23T22:29:11.932952+00:00 heroku[slugc]: Slug compilation finished 2013-04-23T22:29:12.858139+00:00 heroku[web.1]: Stopping all processes with SIGTERM 2013-04-23T22:29:13.194707+00:00 app[web.1]: I, [2013-04-23T22:29:13.194495 #2] INFO -- : reaped #<Process::Status: pid 5 exit 0> worker=0 2013-04-23T22:29:13.194902+00:00 app[web.1]: I, [2013-04-23T22:29:13.194747 #2] INFO -- : master complete 2013-04-23T22:29:14.312014+00:00 heroku[web.1]: Process exited with status 0 2013-04-23T22:29:14.338943+00:00 heroku[web.1]: Starting process with command `bundle exec unicorn -p 25281 -E $RACK_ENV` 2013-04-23T22:29:15.717687+00:00 app[web.1]: I, [2013-04-23T22:29:15.717387 #2] INFO -- : listening on addr=0.0.0.0:25281 fd=7 2013-04-23T22:29:15.717916+00:00 app[web.1]: I, [2013-04-23T22:29:15.717861 #2] INFO -- : worker=0 spawning... 2013-04-23T22:29:15.721526+00:00 app[web.1]: I, [2013-04-23T22:29:15.721385 #2] INFO -- : master process ready 2013-04-23T22:29:15.723623+00:00 app[web.1]: I, [2013-04-23T22:29:15.723389 #5] INFO -- : worker=0 spawned pid=5 2013-04-23T22:29:15.723805+00:00 app[web.1]: I, [2013-04-23T22:29:15.723750 #5] INFO -- : Refreshing Gem list 2013-04-23T22:29:16.305653+00:00 heroku[web.1]: State changed from starting to up 2013-04-23T22:29:16.891063+00:00 app[web.1]: I, [2013-04-23T22:29:16.890882 #5] INFO -- : worker=0 ready ``` After that, I tried to load the homepage in my browser, and these are the only two extra lines that appeared in the logs: ``` 2013-04-23T22:29:26.685651+00:00 heroku[router]: at=info method=GET path=/ host=blofs.herokuapp.com fwd="46.65.16.44" dyno=web.1 connect=2ms service=156ms status=500 bytes=706 2013-04-23T22:29:26.973073+00:00 heroku[router]: at=info method=GET path=/favicon.ico host=blofs.herokuapp.com fwd="46.65.16.44" dyno=web.1 connect=1ms service=5ms status=200 bytes=0 ``` Does anyone have any idea what the problem could be? Other things that might be relevant: - I've disabled the standard Heroku Postgres addon and am using the ClearDB MySQL addon instead. - I've tried manually changing the `DATABASE_URL` environment variable from the previous value (which was still the postgres URL even after I removed the postgres addon) to the same value as the `CLEARDB_DATABASE_URL` var (the mysql one). This didn't seem to make any difference. - I have added `ruby '2.0.0'` to my Gemfile, and added `mysql2` and `unicorn` gems. And I've used the `config/unicorn.rb` suggested at https://devcenter.heroku.com/articles/rails-unicorn - My Procfile only contains: `web: bundle exec unicorn -p $PORT -E $RACK_ENV` - It works fine locally with `foreman start`. - Other than the modifications described above, it's basically an out-of-the-box app created with `rails new`.