"Identifier types can only be instantiated as direct child of a model type"

[Dukeman330]

I've got 2 models:

const UserBase = types
  .model('UserBase', {
    id: types.identifierNumber,
    firstName: types.maybeNull(types.string),
    lastName: types.maybeNull(types.string),
    profile: types.frozen(),
    loadProfileState: types.optional(LoadingState, {}),
  })

const ReadOnlyLogStore = types
  .model('ReadOnlyLogStore', {
    id: types.maybeNull(types.identifierNumber),
    programCards: types.optional(types.array(FeedCard), []),
    programEvents: types.optional(types.array(ProgramEvent), []),
    nextProgramEventDate: types.maybeNull(types.string),
    trackers: types.optional(types.array(Tracker), []),
    loadProgramDataState: types.optional(LoadingState, {}),
    saveProgramDataState: types.optional(LoadingState, {}),
  })

When I try to compose them with:
const User = types.compose(UserBase, ReadOnlyLogStore).named('User');

and instantiate the model with:
const u = User.create({ id, firstName, lastName });

I get the error:
"Identifier types can only be instantiated as direct child of a model type"

What does this mean? Where do I appear to be instantiating an identifier type? If I leave ReadOnlyLogStore out of the composed User object I'm able to instantiate it without issue.