Having trouble with changing weekdays in Kotlin

[kwanggy]

calendarView.weekendDays requires (Mutable)Set<Long>
so I declared mutable set and pass it to weekendDays like this

val set = mutableSetOf<Long>()
        set.add(Calerndar.SATURDAY)  // I also tried WeekDay.SATURDAY
        calendarView.weekendDays = set

However it doesn't recognize saturday as weekendDays. So I researched source code little bit and I found this snippet.
MonthAdapter.java

private void setDaysAccordingToSet(Set<Long> days, DayFlag dayFlag) {
        if (days != null && !days.isEmpty()) {
            for (Month month : months) {
                for (Day day : month.getDays()) {
                    switch (dayFlag) {
                        case WEEKEND:
                            day.setWeekend(days.contains(day.getCalendar().get(Calendar.DAY_OF_WEEK)));
                            break;

days.contains(day.getCalendar().get(Calendar.DAY_OF_WEEK)) returns false everytime, even though Set<Long> days set has "7" and day.getCalendar().get(Calendar.DAY_OF_WEEK)value is 7.
I think it's because "7" in Set<Long> days is Long type and day.getCalendar().get(Calendar.DAY_OF_WEEK) is Int type. It works when i do days.contains((long)day.getCalendar().get(Calendar.DAY_OF_WEEK))

Can anyone help me with this?