Check subscription validity using flutter_inapp_purchase dependency?

I used the flutter_inapp_purchase dependency to make the in-app purchase in the flutter mobile app. But, there is a problem with the chcekSubscribed method. It returns a casting error while using it.

I have changed the existing method and created a custom one. as follows:

Future<bool> checkSubscriptionStatus({
required String sku,
Duration duration = const Duration(days: 30),
Duration grace = const Duration(days: 7),}) async 
{

FlutterInappPurchase _iap = FlutterInappPurchase.instance;

if (Platform.isIOS) {
  var history =
      await (_iap.getPurchaseHistory() );

  if (history == null) {
    return false;
  }

  for (var purchase in history) {
    Duration difference =
        DateTime.now().difference(purchase.transactionDate!);
    if (difference.inMinutes <= (duration + grace).inMinutes &&
        purchase.productId == sku) {
      return true;
    }
  }

  return false;
} else if (Platform.isAndroid) {
  var purchases =
      await (_iap.getAvailablePurchases() );

  for (var purchase in purchases!) {
    if (purchase.productId == sku) {
      return true;
    }
  }

  return false;
}
throw PlatformException(
    code: Platform.operatingSystem, message: "platform not supported");

}