对伴生对象的JSON支持

我有一节课看起来有点像那样

import java.time.OffsetDateTime

import spray.json._
import DefaultJsonProtocol._

sealed trait Person {

  def firstName: String

  def country: String

  def lastName: String

  def salary: Option[BigDecimal]
}

case class InternalPerson(
                     firstName: String,
                     country: String,
                     lastName: Option[BigDecimal],
                     salary: Option[BigDecimal]
                    ) extends Person

object Person {
  def fromName(name: Name, country: String, salary: Option[BigDecimal]): Person = {
    InternalPerson(
              firstName = name.firstName,
              lastName = name.lastName,
              country = country,
              salary = salary
              )
  }
}

object PersonJsonProtocol extends DefaultJsonProtocol {
  implicit val personFormat = jsonFormat4(Person.apply)
}

spray.json._ 从其他课程中我得到:

Note: implicit value personFormat is not applicable here because it comes after the application point and it lacks an explicit result type

value apply is not a member of object of Person

你知道如何让Json支持scala中扩展trait的伴随对象吗?