It looks like none of the uses for Mappings#asList is ready to handle nulls.
So far there are 6 usages of the method, however, it is puzzling if NPE is expected behavior there.
I'm inclined to create two variations of the method:
1) List<@Nullable Integer> asList(...) – keep the current method current one
2) List<@NonNull Integer> asList(...) – throw exceptions on invalid mappings