# File lib/chef/exceptions.rb, line 113
      def initialize(message_or_expansion=NULL)
        @expansion = nil
        case message_or_expansion
        when NULL
          super()
        when String
          super
        when RunList::RunListExpansion
          @expansion = message_or_expansion
          missing_roles = @expansion.errors.join(', ')
          super("The expanded run list includes nonexistent roles: #{missing_roles}")
        end
      end