Boeing said a new manufacturing flaw on its best-selling 737 Max will delay deliveries of its best-selling aircraft, the latest setback as the company tries to hand over more planes.

The company said it found fastener holes on the aft pressure bulkhead on some 737 planes were improperly drilled. Spirit Aerosystems, which makes the fuselages, said that because it “uses multiple suppliers for the aft pressure bulkhead, only some units are affected.”

“This issue will impact near-term 737 deliveries as we conduct inspections to determine the number of airplanes affected, and complete required rework on those airplanes,” Boeing said. It will continue delivering 737 Maxes that are not affected by the issue.

The defect is the latest in a string of manufacturing flaws Boeing has disclosed on the Max and in other programs while it tries to ramp up production to meet strong demand from airlines short on planes during a travel boom. Last month, the company said it is transitioning to a production rate of 38 a month from 31.

Boeing didn’t say whether the new issue would change its forecast to deliver between 400 and 450 Max jets this year.

Spirit Aerosystems said it would continue to deliver fuselages to Boeing.

“We are working closely with our customer to address any impacted units within the production system and address any needed rework,” Spirit Aerosystems said in a statement. “Based upon what we know now, we believe there will not be a material impact to our delivery range for the year related to this issue.”

This year through July, Boeing handed over 309 planes to customers, behind the 381 rival Airbus planes delivered in the same period.

The company said the issue, reported earlier by The Air Current, was not related to flight safety and that airlines can continue flying the planes. Boeing added that it has notified the Federal Aviation Administration.

Boeing shares were down about 3% in after-hours trading.