The head of the Federal Aviation Administration said a software problem will delay its certification of the Boeing 737 Max 10 as the agency assesses whether it poses a safety issue, a potential setback for the manufacturer as it seeks to complete government approval of its best-selling family of aircraft.

Boeing had expected to receive a green light from its regulator on the years-delayed planes “very soon,” CEO Kelly Ortberg told investors earlier this month. But Boeing said Saturday it had last month flagged a software glitch on some 737 Max aircraft that could affect certain landing procedures.

“We haven’t concluded whether this is a safety-of-flight issue or not, but we will be delaying the 10 … until we’re satisfied that we don’t have an issue here,” FAA Administrator Bryan Bedford said at a press conference on air traffic modernization in Washington, D.C., on Monday.

Boeing shares were down more than 6% in afternoon trading, extending earlier losses after Bedford’s comments.

“We continue to follow the lead of the FAA as we work through the certification process,” Boeing said in a statement.  

Boeing said over the weekend that the issue could occur with a vertical navigation system after an aircraft misses an approach and has to go around and line up to land again, such as in situations when there is an obstruction on the runway, for example.

Bedford said the FAA approved the Max 7 last month, the smallest of the aircraft in the family, and that Boeing had fixed a previous bug in the system but “unexpectedly … introduced a new bug.”

U.S. airlines said they don’t have any Max aircraft with the bug and have reverted to older software without the problem, if needed.