Coffee Mug Warmer,Coffee Warmer,Coffee Warmer for Desk, Smart Constant Temperature Coaster Usb Powered Milk Cup Heater Pad for Home Kitchen Office

I did not note before I bought it that this works only with the least common type of mug. It must have an absolutely flat bottom. All of mine are the other kind, with a slight ridge around the circumference. I bought a 2-in metal washer which helps, but I wish they would either include something or just make a mug warmer that works with the more common type of mug.