循环中的歧义

我得到的错误是"System.ArgumentOutOfRangeException未处理。"它要求月份在1到12之间。但看看调试器中的变量，它等于1，还有一个Debug.Writeline.

    int month, year, total;
    total = 0;
    DateTime dayToFind;
    for (year = 1001; year < 1201; year++){
        for (month = 1; month < 12; month++){
            dayToFind = new DateTime(year, month, DateTime.DaysInMonth(month, year));
            // The error points at the last occurance of month above.
            total = (dayToFind.DayOfWeek == DayOfWeek.Monday) ? 1 : 0;
        }
    }