The Conrad Seoul apologized Thursday amid an investigation after the five-star hotel’s buffet restaurant accidentally served detergent to a guest instead of syrup.

The luxury hotel, part of Hilton’s international high-end brand Conrad Hotels & Resorts, said it “sincerely apologizes” to the customer for the “major inconvenience” and vowed to maintain health and safety rules.

“We are actively cooperating with the authorities on the investigation,” the statement read.

The case, involving allegations that the hotel and four staff members committed professional negligence resulting in injury and breached the Food Sanitation Act, was sent to the prosecution by Seoul’s Yeongdeungpo Police Station last month, local media have reported.