All sub class of Sealed class is immutable. Sealed class should be immutable

[meowpunch]

@RequiredArgsConstructor
public sealed class A permits B {
  private final int i;
}

public final class B extends A {
  B(int i) {
    super(i);
  }
}

I got error An unhandled error occurred while analysing the class

Expected: A is immutable