Problems also arose between Diogo and the Portuguese settlers at Sao Tome known as Tomistas. According to a treaty between Kongo and Portugal, the latter were only to trade within the former's realm for slaves. That meant the Portuguese were restricted to the slaves offered by King Diogo or those he authorized to sell slaves. Every year the Tomistas would come with 12 to 15 ships to carry back between 400 and 700 slaves (5,000–10,000 slaves a year). This was not enough to take advantage of Kongo's ever-growing supply of slaves thanks to wars on its eastern frontier. The captains would try to overload their cargos, resulting in revolts. However, the factor that actually broke the deal was the Tomista habit of sailing upriver to the Malebo Pool to purchase slaves from BaTeke traders who were increasingly taken with European goods over the nzimbu shells the manikongo offered them. Enraged by this breach of contract, King Diogo broke off relations in 1555 and expelled 70 or so Portuguese living in his realm (many of whom had lived there for a long time and had African wives and mixed-race children).