It sounds like the problem is the generic wild car...

# announcements

[Ruckus]

It sounds like the problem is the generic wild card. Kotlin's immutable list interface is implemented (as far as Java is concerned) as the ? extends format you see above. If you change it to MutableList<MySealedClass> or java.util.List<MySealedClass> you should be okay. See @orangy's answer below.