There is provided mechanisms for reporting a faulty antenna port at a transmitting radio device. A method is performed by a receiving radio device. The method comprises receiving at least one reference signal transmitted at an antenna port of the transmitting radio device. The method comprises determining whether the antenna port is deemed faulty or not by subjecting the at least one reference signal to an evaluation criterion. The method comprises transmitting, when the antenna port is deemed faulty, a report to the transmitting radio device. The report indicates that the antenna port is deemed faulty.