Silent Failure when starting Unicorn on Heroku

Switched my RoR (3.1.3, 1.9.2) app from Thin to Unicorn (4.6.2). It ran fine in development (Mac OSX), but when deploying to production on Heroku, it fails silently (no backtrace, no error detail). Log looks like:

Starting process with command `bundle exec unicorn start -p 6069 -c ./config/unicorn.rb`   /app/vendor/bundle/ruby/1.9.1/gems/unicorn-4.6.2/lib/unicorn/configurator.rb:634:in `parse_rackup_file': rackup file (start) not readable (ArgumentError)   from /app/vendor/bundle/ruby/1.9.1/gems/unicorn-4.6.2/lib/unicorn/configurator.rb:77:in `reload'   from /app/vendor/bundle/ruby/1.9.1/gems/unicorn-4.6.2/lib/unicorn/configurator.rb:68:in `initialize'   from /app/vendor/bundle/ruby/1.9.1/gems/unicorn-4.6.2/lib/unicorn/http_server.rb:108:in `new'   from /app/vendor/bundle/ruby/1.9.1/gems/unicorn-4.6.2/lib/unicorn/http_server.rb:108:in `initialize'   from /app/vendor/bundle/ruby/1.9.1/gems/unicorn-4.6.2/bin/unicorn:126:in `new'   from /app/vendor/bundle/ruby/1.9.1/gems/unicorn-4.6.2/bin/unicorn:126:in `<top (required)>'   from /app/vendor/bundle/ruby/1.9.1/bin/unicorn:19:in `load'   from /app/vendor/bundle/ruby/1.9.1/bin/unicorn:19:in `<main>' Process exited with status 1 State changed from starting to crashed

Procfile:     web: bundle exec unicorn start -p $PORT -c ./config/unicorn.rb     sidekiq: bundle exec sidekiq -c 10

unicorn.rb:     worker_processes Integer(ENV["WEB_CONCURRENCY"] || 3)     timeout 15     preload_app true

    before_fork do |server, worker|

      Signal.trap 'TERM' do         puts 'Unicorn master intercepting TERM and sending myself QUIT instead'         Process.kill 'QUIT', Process.pid       end

      defined?(ActiveRecord::Base) and         ActiveRecord::Base.connection.disconnect!     end

    after_fork do |server, worker|

      Signal.trap 'TERM' do         puts 'Unicorn worker intercepting TERM and doing nothing. Wait for master to sent QUIT'       end

      defined?(ActiveRecord::Base) and         ActiveRecord::Base.establish_connection     end

I have no idea where to even begin to troubleshoot this problem. Any thoughts?

Problem turned out to be in my Procfile. Changed "bundle exec unicorn start" to "bundle exec unicorn" and problem went away. Not sure where I found the "start" syntax. Oh well.