Estou executando a seguinte consulta e recebi o erro "Subconsulta retorna mais de 1 linha"

My Query is

SELECT pkTicketID,
       TicketReplyDateAdded,
       TIMESTAMPDIFF(DAY, TicketReplyDateAdded, now()) as NumberOfDays
  FROM tickets as T
    LEFT JOIN ticket_replies as TR ON T.fkTicketReplyID=TR.pkTicketReplyID
  WHERE 1 AND T.fkEmployeeID = '4'
    AND (SELECT TIMESTAMPDIFF(DAY, TicketReplyDateAdded, now()) as NumberOfDays
           FROM tickets as T
             LEFT JOIN ticket_replies as TR
               ON T.fkTicketReplyID=TR.pkTicketReplyID
        ) = 7
    AND T.TicketStatus = 'Replied'
  ORDER BY pkTicketReplyID DESC

Obrigado pela ajuda. Umar