Csúnya multithreading hiba

Double checked lockingnak indult, de bug lett belőle. Mit rontottam el?

private static volatile Dictionary> holidayDays;
private static readonly object staticLock = new object();

private Dictionary> GetHolidayDays()
{
if (holidayDays == null)
{
lock (staticLock)
{
if (holidayDays == null)
{
holidayDays = new Dictionary>();
FillTradingHours(holidayDays, “HOL”);
}
}
}
return holidayDays;
}