Union Finance Minister Arun Jaitley today warned black money holders of serious trouble if they fail to come clean.Mr. Jaitley said people with undisclosed assets should use the limited period income declaration window, beginning Wednesday.The four-month window under the Income Declaration Scheme 2016, which opens on June 1, allows domestic black money holders to declare their ill-gotten wealth by paying a tax and penalty totaling 45 per cent.In an interview to PTI Union Finance Minister who is on a six-day trip to Japan to invite investments, said the compliance window is open to not only the companies but also the people having undisclosed income.