Diabetic Cat

I have a diabetic cat. He takes insulin twice daily. Does anyone know why he keeps urinating in his cat bed? Even when I wash it, he eventually goes right back to it and urinates in it. He is only urinating in the litter box otherwise. Is there anyway (short of getting rid of catbeds all together) that I can prevent him from urinating in the bed? This is the second catbed in which he has done that. Thanks!