scala - Scala：隐式 + 覆盖 (+ReactiveMongo)

Question

有代码：

object MarketTrigger extends DbHandler[MarketTrigger]{
  implicit object BSONDateTimeHandler extends BSONHandler[BSONDateTime, DateTime] {
    def read(time: BSONDateTime) = new DateTime(time.value)
    def write(jdtime: DateTime) = BSONDateTime(jdtime.getMillis)
  }

  implicit val mtFormat = Macros.handler[MarketTrigger]
  override implicit val format = Macros.handler[MarketTrigger]
  ...

我有Implicit org.joda.time.DateTime for 'value till' not found第二个隐式 val 的错误，而第一个没问题。对我来说有点奇怪的行为。这是怎么回事？