Option not working for user-defined types

The following unit test does not run:
    `it should "parse Option[MyNumber]" in {
    object X extends CellParsers {
      import MyNumberParser._
      implicit val optionalPrincipalParser: CellParser[Option[MyNumber]] = cellParserOption[MyNumber]
    }
    import X._
    implicitly[CellParser[Option[MyNumber]]].convertString("1") shouldBe Some(1)
  }
`