Cookie secret error HTTP 500

[adrianshort]

Rack::Flash seems to be throwing this error, giving HTTP 500 and a blank page to the user:

SECURITY WARNING: No secret option provided to Rack::Session::Cookie.
This poses a security threat. It is strongly recommended that you
provide a secret to prevent exploits that may be possible from crafted
cookies. This will not be supported in future versions of Rack, and
future versions will even invalidate your existing user cookies.

Called from: /app/vendor/bundle/ruby/1.9.1/gems/rack-1.5.2/lib/rack/builder.rb:86:in `new'.