I have a class that is used to be serialized to Json without problems with kotlin 1.2 (0.6.2 with no customized serializers). after migrating to 1.3 (0.8,3), it stopped working, complaining about either some enum class or java singletonMap can't be serialized. is this expected? I don't see any mentioning about backward imcompatibility in the 1.3 doc.