intertwingly

Not rack’s fault, but Sinatra hasn’t released in a while.  Problem has been known since July, and a fix was merged into master in August.  One possible workaround has been posted.  An alternate workaround:

module Rack
  class ShowExceptions
    alias_method :old_pretty, :pretty
    def pretty(*args)
      result = old_pretty(*args)
      def result.join; self; end
      def result.each(&block); block.call(self); end
      result
    end
  end
end