Erik Spoelstra blamed himself for a “horrendous mistake” after calling a timeout the Miami Heat didn’t have with 1.1 seconds left in overtime Tuesday night, a technical foul that gave the Detroit Pistons the tiebreaking free throw.

Malik Beasley made it and the Pistons went on to beat the Heat 123-121 on the opening night of NBA Cup play.

“I made just a serious mental error there at the end,” Spoelstra said . “That’s on me. I feel horrible about it.”

The Heat had a 121-119 lead after Tyler Herro’s basket with 1.8 seconds remaining. But after a timeout, the Pistons ran a well-executed inbounds play, with Cade Cunningham throwing a lob pass that Jalen Duren slammed down.

Spoelstra quickly and angrily stormed onto the court and motioned for a timeout.

“There’s really no excuse for that,” Spoelstra said. “I’m 17 years in. We had talked about it in the huddle, I knew that we didn’t have anything. I just got emotional and reactive on that and I made just a horrendous mistake there at the end. It’s a shame.”

Spoelstra has won two NBA championships, was an assistant coach on the U.S. team that won an Olympic gold medal this summer, and was the overwhelming choice of executives who responded in the NBA.com GM Survey as the NBA’s best coach, earning 69% of the vote.

But he made a rookie mistake Tuesday that negated a brilliant performance by Herro, who scored 40 points and made 10 3-pointers.

“You don’t want it to come down to a mental error like that,” Spoelstra said, before a lengthy pause. “Yeah, you would have just liked to see this go double overtime. That’s the kind of game it felt like. It deserved to go double OT and not have somebody get in the way of that, and unfortunately even as a veteran coach I got in the way of that.”