    # 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