On May 16, 1815, Representative-Elect Jonathan Williams (DR) who'd been elected for Pennsylvania's 1st district, died before the start of the 14th Congress. A special election was held on October 10 of that year to fill the vacancy left by his death.
Pennsylvania's 1st congressional district special election, 1815 Wikipedia
Williams had been the sole Democratic-Republican elected to Pennsylvania's 1st district (a plural district with 4 seats), and so with Sergeant's win, all four of the 1st district's seats were held by Federalists. Sergeant took his seat in the Congress on December 6, 1815