我的网络应用程序使用的是Rack 1.4.0，Sinatra v1.2.6，Thin v1.2.11，Ruby v1.9.2p180
导航到某个页面我在日志中收到此错误：
TypeError - can't convert nil into String:
/usr/local/lib/ruby/gems/1.9.1/gems/rack-1.4.0/lib/rack/session/cookie.rb:152:in `hexdigest'
/usr/local/lib/ruby/gems/1.9.1/gems/rack-1.4.0/lib/rack/session/cookie.rb:152:in `generate_hmac'
/usr/local/lib/ruby/gems/1.9.1/gems/rack-1.4.0/lib/rack/session/cookie.rb:109:in `unpacked_cookie_data'
/usr/local/lib/ruby/gems/1.9.1/gems/rack-1.4.0/lib/rack/session/cookie.rb:99:in `extract_session_id'
/usr/local/lib/ruby/gems/1.9.1/gems/rack-1.4.0/lib/rack/session/abstract/id.rb:43:in `load_session_id!'
/usr/local/lib/ruby/gems/1.9.1/gems/rack-1.4.0/lib/rack/session/abstract/id.rb:32:in `[]'
/usr/local/lib/ruby/gems/1.9.1/gems/rack-1.4.0/lib/rack/session/abstract/id.rb:262:in `current_session_id'
/usr/local/lib/ruby/gems/1.9.1/gems/rack-1.4.0/lib/rack/session/abstract/id.rb:268:in `session_exists?'
/usr/local/lib/ruby/gems/1.9.1/gems/rack-1.4.0/lib/rack/session/abstract/id.rb:107:in `exists?'
/usr/local/lib/ruby/gems/1.9.1/gems/rack-1.4.0/lib/rack/session/abstract/id.rb:122:in `load_for_read!'
/usr/local/lib/ruby/gems/1.9.1/gems/rack-1.4.0/lib/rack/session/abstract/id.rb:59:in `[]'
/var/www/phrogz.net/routes/test.rb:8:in `logged_in?'
test.rb第8行中的代码是：
session[ :foo ] == "BAR"
有关升级/降级以解决此问题的任何建议吗？