title: When I try adding rack-flash to my sinatra app it stops working
body: <p>I've been using <code>Rack::Flash</code> for a while to display flash notices on responses in my Sinatra apps. Since I upgraded to Sinatra 1.3.2, the app crashes, logging the following whenever I make a request to the server:</p>
  
  <pre><code>!! Unexpected error while processing request: undefined method `&lt;&lt;' for nil:NilClass
  </code></pre>
  
  <p>The simplified code looks like this:</p>
  
  <pre><code>require 'sinatra'
  require 'rack-flash'
  
  enable :sessions
  use Rack::Flash
  
  get "/" do
    "Hello World"
  end
  </code></pre>
  
  <p>Any idea why this could be happening?</p>