Misleading error message when trying to use _.into.transform() or _.to in a given Transformer definition

I must be dense but the following:

  final case class A(a: Int)
  final case class B(b: Int)
  given Transformer[A, B] =
    _.into[B]
      .transform(Field.renamed(_.b, _.a))

gives me The path segment 'b' is not valid as it is not a field of a case class or an argument of a function @ B

What am I missing?

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