Negotiations with the king of Poland, Zygmunt the Old, began in March 1525, and on April 8, 1525 the Treaty of Kraków was signed, according to which Albert became the Duke of secular Prussia, which he held as a fief from the Polish king. The formal investiture of Albert by Zygmunt took place two days later in the Prussian Homage. By this time Königsberg was already known as a Lutheran city, with its bishop, George of Polentz holding the distinction of being the first Catholic bishop who officially converted to Lutheranism.