MINNEAPOLIS (AP) — The NFL is reminding teams they're not allowed to use artificial means to keep footballs warmer and softer during games.

Fox's broadcast of the Carolina-Minnesota game on Sunday showed a clip of an unidentified sideline attendant holding two balls in front of one of the heaters teams use on cold days. The temperature at kickoff for the game was 12 degrees.

Vice president of officiating Dean Blandino said on NFL Network on Monday that once the league noticed, both the Panthers and Vikings were warned of the rule against using a heater on the football to make it easier to handle during a game. Blandino also said the league will remind all teams that the practice is not allowed.