If the rest of the plant looks healthy you're plant is probably susceptible to low humidity in the house. That's common this time of year with all the doors shut tight and higher artificial heat settings. You could put a humidifier in the room, spritz the leaves with fine spray everyother day or just wait until spring aririves. Keep in mind that in-door plants with variegated leaves need quite a bit more sunshine that plain ones. If this type of plant leaf doesn't receive quite a bit of sunshine, they tend to go off color and appear dull compared to the summer time condition. Best of luck.