# File lib/email_spec/matchers.rb, line 351
      def failure_message_when_negated
        if @expected_value.is_a?(String)
          "expected the headers not to include '#{@expected_name}: #{@expected_value}' but they were #{mail_headers_hash(@given_header).inspect}"
        else
          "expected the headers not to include '#{@expected_name}' with a value matching #{@expected_value.inspect} but they were #{mail_headers_hash(@given_header).inspect}"
        end
      end