The airplane maker recommended airlines undertake the inspection after an international operator discovered a bolt with a missing nut while performing routine maintenance, said a spokesman for the US Federal Aviation Administration.

“The FAA will consider additional action based on any further discovery of loose or missing hardware,” said the FAA spokesman, adding that Boeing had also found an undelivered aircraft with a bolt not properly tightened.

The Boeing plane “has been remedied,” said a company spokesman.

“Out of an abundance of caution, we are recommending operators inspect their 737 MAX airplanes and inform us of any findings. We informed the FAA and our customers and will continue to keep them aware of the progress.”

The inspection only lasts about two hours, but the issue comes on the heels of other manufacturing and production problems that forced the company to lower its delivery targets for the 737 MAX this year.

Shares of Boeing were down 1.2% at midday.