South Carolina's first effort at nullification occurred in 1822. Its planters believed that free black sailors had assisted Denmark Vesey in his planned slave rebellion. South Carolina passed a Negro Seamen Act, which required that all black foreign seamen be imprisoned while their ships were docked in Charleston. The UK strongly objected, especially as it was recruiting more Africans as sailors. What was worse, if the captains did not pay the fees to cover the cost of jailing, South Carolina would sell the sailors into slavery. Other Southern states also passed laws against free black sailors.