title: NoMethodError: undefined method << for nil:NilClass
body: <p>I'm learning sinatra and I get the following error</p>
  
  <pre><code>Boot Error
  
  Something went wrong while loading recall.rb
  NoMethodError: undefined method `&lt;&lt;' for nil:NilClass
  
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/rack-flash-0.1.2/lib/rack/flash.rb:11:in `use'
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/shotgun-0.9/bin/shotgun:128:in `block (2 levels) in &lt;top (required)&gt;'
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/shotgun-0.9/lib/shotgun/loader.rb:104:in `instance_eval'
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/shotgun-0.9/lib/shotgun/loader.rb:104:in `block in assemble_app'
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/rack-1.4.1/lib/rack/builder.rb:51:in `instance_eval'
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/rack-1.4.1/lib/rack/builder.rb:51:in `initialize'
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/shotgun-0.9/lib/shotgun/loader.rb:103:in `new'
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/shotgun-0.9/lib/shotgun/loader.rb:103:in `assemble_app'
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/shotgun-0.9/lib/shotgun/loader.rb:86:in `proceed_as_child'
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/shotgun-0.9/lib/shotgun/loader.rb:31:in `call!'
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/shotgun-0.9/lib/shotgun/loader.rb:18:in `call'
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/shotgun-0.9/lib/shotgun/favicon.rb:12:in `call'
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/shotgun-0.9/lib/shotgun/static.rb:14:in `call'
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/rack-1.4.1/lib/rack/builder.rb:134:in `call'
  /Users/nikhil/.rvm/gems/ruby-1.9.3-p194/gems/rack-1.4.1/lib/rack/handler/webrick.rb:59:in `service'
  /Users/nikhil/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/webrick/httpserver.rb:138:in `service'
  /Users/nikhil/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/webrick/httpserver.rb:94:in `run'
  /Users/nikhil/.rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/webrick/server.rb:191:in `block in start_thread'
  </code></pre>
  
  <p>I'm not able to decipher it as id doesn't point to any of my source files.</p>
  
  <p>The error arose when I followed <a href="http://net.tutsplus.com/tutorials/ruby/singing-with-sinatra-the-encore/" rel="nofollow">this part</a> of a tutorial where they talk about notices and error messages. 
  The main file for my app includes the following gems. </p>
  
  <pre><code>require "sinatra"
  require "data_mapper"
  require "rack-flash"
  require "sinatra/redirect_with_flash"
  </code></pre>