Burger King issued an apology Friday, after a plastic wrapper was found inside a burger sold at one of its stores in Sejong.

“Burger King would like to sincerely apologize to the customer for the inconvenience,“ the US burger chain said in a statement.

”We are currenly investigating how the substance got into the food. Burger King will increase efforts to improve the quality of our products and services.“

On Thursday, a male customer told local media he found a sheet of plastic wrapper in a burger bought from a franchise burger joint in Jochiwon, Sejong, on his way home. He filed a complaint the next day.

The store in question admitted it was their fault and said it suspects the patties were cooked without the plastic wrapper removed. The customer reportedly received a refund from the store.

This is not the first time a foreign substance has been found in burgers served at a major franchise.

Early this month, an earwig was found from a burger made at a branch of a different fast food chain in Suwon, Gyeonggi Province. The Korea Food and Drug Administration is set to issue a correction order to the franchise sometime next month.