Deficiency in the enzyme lactase

Question: Mr. Chow has difficulties digesting milk products due to a deficiency in the enzyme lactase. His doctor has provided him with a solution of lactase to consume every time he eats a dairy product. Mr. Chow decides he wants some hot chocolate. He boils his lactase with his milk to save time. His hot chocolate causes him a severe stomachache, so he calls his doctor to complain. Why did his lactase fail to work?