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.