I want to create a function that calculate expense total and income total based on provided month…and if year is not provided it should calculate based on current year,

so i want to give default value to current year, but its showing error

I want to create a function that calculate expense total and income total based on provided month…and if year is not provided it should calculate based on current year,

so i want to give default value to current year, but its showing error at design time

Map<String, dynamic> monthBalanceCard({required int month, int year=DateTime.now().year}) {
    final expenseTransactions = _allTransactions
        .where((element) => element.isexpense)
        .toList()
        .where((element) =>
            element.date.year == year && element.date.month == month);
    final incomeTransactions = _allTransactions
        .where((element) => element.isexpense = false)
        .toList()
        .where((element) =>
            element.date.year == year && element.date.month == month);

    double expenseTotal = expenseTransactions.fold(
        0, (previousValue, element) => previousValue + element.amount);

    double incomeTotal = expenseTransactions.fold(
        0, (previousValue, element) => previousValue + element.amount);

    double balance = expenseTotal - incomeTotal;

    return {
      'Expense': expenseTotal,
      'Income': incomeTotal,
      'Balance': balance,
    };
  }