In 1538, in the Treaty of Nagyvárad, Ferdinand induced the childless Zápolya to name him as his successor. But in 1540, just before his death, Zápolya had a son, John II Sigismund, who was promptly elected King by the Diet. Ferdinand invaded Hungary, but the regent, Frater George Martinuzzi, Bishop of Várad, called on the Ottomans for protection. Suleiman marched into Hungary (see Siege of Buda (1541)) and not only drove Ferdinand out of central Hungary, he forced Ferdinand to agree to pay tribute for his lands in western Hungary.