Slick 2.0.1: How to map from Set[String] to Option[String]

slick.scala

implicit val SetOfStrings = MappedColumnType.base[Set[String], Option[String]] (
  values => values match {
    case values if values.nonEmpty => Some(values.toSeq.sorted.mkString("\n"))
    case _ => None
  },
  str => str.map(_.split('\n').toSet).getOrElse(Set.empty)
)
// could not find implicit value for evidence parameter of type SchemaBase.this.driver.BaseColumnType[Option[String]]