Testing my bloc fails when using yield and Either in the mapEventToState

I'm following a course on clean architecture and I'm stuck with the following error.

Both of the following tests fails.

test('should emit [Error] when the input is invalid', () async {
  final tNumberString = '1';
  when(mockInputConverter.stringToUnsignedInteger(any))
    .thenReturn(Left(InvalidInputFailure()));

  final expected = [
    Empty(),
    Error(message: invalidInput),
   ];
   expectLater(bloc.state, emitsInOrder(expected));

   bloc.add(GetTriviaForConcreteNumberEvent(tNumberString));
});

My NumberTriviaBlock is as follows

part 'number_trivia_event.dart';
part 'number_trivia_state.dart';

const String serverFailureMessage = 'Server Failure';
const String cacheFailureMessage = 'Cache Failure';
const String invalidInput =
    'Invalid input - the number should be a positive integer';

class NumberTriviaBloc extends Bloc<NumberTriviaEvent, NumberTriviaState> {
  NumberTriviaBloc(
      {@required GetConcreteNumberTrivia concrete,
      @required GetRandomNumberTrivia random,
      @required this.inputConverter})
      : assert(concrete != null),
        assert(random != null),
        assert(inputConverter != null),
        _getConcreteNumberTrivia = concrete,
        _getRandomNumberTrivia = random,
        super(Empty());

  final GetConcreteNumberTrivia _getConcreteNumberTrivia;
  final GetRandomNumberTrivia _getRandomNumberTrivia;
  final InputConverter inputConverter;

  @override
  Stream<NumberTriviaState> mapEventToState(
    NumberTriviaEvent event,
  ) async* {
    if (event is GetTriviaForConcreteNumberEvent) {
      final inputEither =
          inputConverter.stringToUnsignedInteger(event.numberString);

      yield* inputEither.fold(
        (l) async* {
          yield Error(message: invalidInput);
        },
        (r) => throw UnimplementedError(),
      );
    }
  }
}

Im not that familiar with the bloc patern and the testing mechanisms of streams and states. Im guessing somehow the code isn't executed correctly because the fail message is as follows

ERROR: Expected: should do the following in order:
emit an event that Empty:<Empty>
emit an event that Error:<Error>
Actual: Empty:<Empty>
Which: was not a Stream or a StreamQueue