Play Framework Json 직렬화 실패

암시 적 Json이 다음과 같이 읽고 씁니다.

  implicit val userJsonWrites = new Writes[User] {
    override def writes(user: User): JsValue = Json.obj(
      idKey -> Json.toJson(user._id),
      firstNameKey -> Json.toJson(user.firstName),
      lastNameKey -> Json.toJson(user.lastName),
      emailKey -> Json.toJson(user.email),
      //passKey -> Json.toJson(user.pass),
      addressKey -> Json.toJson(user.address),
      createDateKey  -> Json.toJson(user.createDate),
      activateDateKey  -> Json.toJson(user.activateDate),
      isUserActivatedKey -> Json.toJson(user.isUserActivated),
      verificationDateKey  -> Json.toJson(user.verificationDate)
    )
  }

  implicit val userJsonReads = new Reads[User] {
    override def reads(json: JsValue): JsResult[User] = {
      val user = User(
        _id = (json \ idKey).as[Option[String]],
        firstName =  (json \ firstNameKey).as[String],
        lastName =  (json \ lastNameKey).as[String],
        email =  (json \ emailKey).as[String],
        pass =  (json \ passKey).as[String],
        address = (json \ addressKey).as[Address],
        createDate =  (json \ createDateKey).as[DateTime],
        activateDate =  (json \ activateDateKey).as[Option[DateTime]],
        verificationDate =  (json \ verificationDateKey).as[Option[DateTime]],
        isUserActivated = (json \ isUserActivatedKey).as[Boolean]
      )

      JsSuccess(user)
    }
  }

컴파일 할 때 다음과 같은 오류가 발생합니다.

Error:(84, 32) not enough arguments for method as: (implicit fjs: play.api.libs.json.Reads[Option[String]])Option[String].
Unspecified value parameter fjs.
        _id = (json \ idKey).as[Option[String]],
                               ^
Error:(84, 32) No Json deserializer found for type Option[String]. Try to implement an implicit Reads or Format for this type.
        _id = (json \ idKey).as[Option[String]],
                           ^

내 사용자 개체는 다음과 같습니다.

case class User(
  _id: Option[String],
  createDate: DateTime,
  activateDate: Option[DateTime],
  verificationDate: Option[DateTime],
  email: String,
  pass: String,
  firstName: String,
  lastName: String,
  isUserActivated: Boolean,
  address: Address
)

_id는 실제로 내가 옵션으로 가지고 있어야하는 MongoDB의 ObjectId입니다!