title: Puma caught this error: undefined method `call' for :debug:Symbol
body: <p>My rails app is 5.1.2. Everything was working fine until today I got this error when <code>rails s</code> .</p>
  
  <pre><code>Use Ctrl-C to stop
  2017-08-17 10:49:28 +0800: Rack app error handling request { GET / }
  #&lt;NoMethodError: undefined method `call' for :debug:Symbol
  Did you mean?  caller&gt;
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/activesupport-5.1.2/lib/active_support/deprecation/reporting.rb:21:in `block (2 levels) in warn'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/activesupport-5.1.2/lib/active_support/deprecation/reporting.rb:21:in `each'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/activesupport-5.1.2/lib/active_support/deprecation/reporting.rb:21:in `block in warn'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/activesupport-5.1.2/lib/active_support/deprecation/reporting.rb:20:in `tap'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/activesupport-5.1.2/lib/active_support/deprecation/reporting.rb:20:in `warn'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/activesupport-5.1.2/lib/active_support/deprecation/instance_delegator.rb:20:in `warn'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/activesupport-5.1.2/lib/active_support/deprecation/instance_delegator.rb:27:in `warn'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/railties-5.1.2/lib/rails/application.rb:506:in `validate_secret_key_config!'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/railties-5.1.2/lib/rails/application.rb:247:in `env_config'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/railties-5.1.2/lib/rails/engine.rb:692:in `build_request'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/railties-5.1.2/lib/rails/application.rb:518:in `build_request'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/railties-5.1.2/lib/rails/engine.rb:521:in `call'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/puma-3.9.1/lib/puma/configuration.rb:224:in `call'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/puma-3.9.1/lib/puma/server.rb:602:in `handle_request'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/puma-3.9.1/lib/puma/server.rb:435:in `process_client'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/puma-3.9.1/lib/puma/server.rb:299:in `block in run'
  /Users/user/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/puma-3.9.1/lib/puma/thread_pool.rb:120:in `block in spawn_thread'
  2017-08-17 10:49:28 +0800: Rack app error handling request { GET /favicon.ico }
  #&lt;NoMethodError: undefined method `call' for :debug:Symbol
  Did you mean?  caller&gt;
  </code></pre>
  
  <p>Tried everything I could think of:</p>
  
  <ul>
  <li>Downgrade to rails 5.1.0</li>
  <li>Switched ruby versions to 2.3.3 or 2.3.4</li>
  </ul>
  
  <p>Still no luck.</p>
  
  <p>Appreciate any help. Please let me know if you need any more info.</p>
  
  <p>Thank you!</p>