controller failing to read flash

I'm seeing some pretty strange behavior from the flash in my rails 2.3.8 app. I set the flash in a request, and then redirect. In the next request (the one redirected to), accessing the flash returns an empty hash, but the flash is stored and visible in the session.

(rdb:21) session {:user_credentials=>"...", :session_id=>"...", :user_credentials_id=>1, :flash=>{:notice=>"Twitter status updated"}, :_csrf_token=>"..."} (rdb:21) session[:flash] {:notice=>"Twitter status updated"} (rdb:21) flash {}

Looking at the source for flash in actionpack-2.3.8/lib/ action_controller/flash.rb, the difference is that the key is stored as a string and not a symbol. How could that disparity occur?

# Access the contents of the flash. Use <tt>flash["notice"]</tt> to         # read a notice you put there or <tt>flash["notice"] = "hello"</tt>         # to put a new one.         def flash #:doc:           if !defined?(@_flash)             @_flash = session["flash"] ||             @_flash.sweep           end

          @_flash         end