Boeing Co. is at risk of missing delivery targets for its best-selling 737 Max model after discovering that its largest supplier, Spirit AeroSystems Holdings Inc., improperly drilled holes in a component that helps maintain cabin pressure.

The latest issue for Boeing’s cash-cow jet isn’t a safety threat, the Federal Aviation Administration said Wednesday. But it’s another complication for Boeing as it speeds the manufacturing pace of the 737 family while dealing with supply-chain strains and the aftermath of a strike at Spirit, which builds about 70% of the narrowbody jet frames.

“During factory inspections, we identified fastener holes that did not conform to our specifications in the aft pressure bulkhead on certain 737 airplanes,” Boeing said via email Wednesday, confirming an earlier report by the Air Current.

Boeing shares fell 3.4% in after-hours trading on reports of the new Max issue. The stock had gained 20% this year through Wednesday’s close as demand surges for travel and new jetliners.

The issue will affect near-term 737 deliveries, including a plane going to the Malaysian Airline System, as Boeing conducts inspections and determines how many models were affected and what work they need, the spokesperson said. Boeing is evaluating whether it will still be able to reach its target of delivering between 400 and 450 of the 737-family jets this year.