When doing left join on none numeric data type there will be type miss match error

class Fund private() extends Record[Fund] with KeyedRecord[String] {
...
val id = new StringField(this, 8)
val lcc = new OptionalStringField(this, 8)
}

class Lc private() extends Record[Lc] with KeyedRecord[String] {
...
val lcc = new StringField(this, 8)
val name = new StringField(this, 8)
}

val d = join(fund, lc.leftOuter)((f, l) =>
where(f.id === "xxx")
select(f, l)
on(f.lcc === l.map(_.lcc)))

Error mismatch on last line "on(f.lcc === l.map(_.lcc))"
Due to f.lcc is converted to some sort of StringExpression[String] instead of StringExpression[Option[String]]

current work arrount is "on(Some(f.lcc) === l.map(_.lcc))"

more discussion in
ExampleName