The U.S. has shipped Stingers to Ukraine’s military, which has used them to shoot down Russian aircraft. But there’s only a finite supply as Raytheon has not made Stinger missiles for the U.S. military in nearly two decades.

“We’re going to have to go out and redesign some of the electronics in the missile and the seeker head,” Raytheon Technologies CEO Greg Hayes told investment analysts Tuesday during the company’s quarterly earnings call. “That’s going to take us a little bit of time.”

The company has “a very limited stock of material for Stinger production,” Hayes said, as it’s currently building a missile for a sole international customer.