on leftJoin TypeMapper for T is being applied on Option[T]

[NPCRUS]

having two tables wiith relation A.bId = B.id
on of witch has field of custom type T and therefore provided with TypeMapper for this custom type.

case class A[T[_]](id: T[Int], bId: T[Option[Int]])
object A extends Table[A]

enum Custom {
  case Foo, Bar
}
object Custom {
  given TypeMapper[Custom] = StringType.bimap(_.toString, Custom.valueOf)
}

case class B[T[_]](id: T[Int], custom: T[Custom])
object B extends Table[B]

having in database only records for table A, so in table B there are no records
and performing leftJoin on table B in such way:

A.select.leftJoin(B)(_.bId `=` _.id)

i get following error: enum Custom has no case with name: null
when in fact there are no records of table B and no custom field to perform type mapping on.

Expected result: it shouldn't try to serialize null into Custom, in fact there should be no value to serialize on at all