FutureOr<T?> analyzer compiler mismatch error

main.dart

import 'dart:async' show FutureOr;
import 'dart:math' show Random;

final rand = Random();

void main() async {
  final hi = await maybeString();
  maybeString()?.then((v) => v); // error mismatch here
}

FutureOr<String?> maybeString() {
  if (rand.nextBool()) return null;
  
  return Future.value('hello world');
}