(CNN) -- A Japanese operator of Boeing's 787 Dreamliners said it suffered a "minor" setback while conducting a test flight of an aircraft this month.
All Nippon Airways, the Japanese airline that is the largest operator of Boeing's beleaguered Dreamliner, said an electric distribution panel overheated and blackened during a 787 test flight recently.
The May 4 test flight landed safely, and the cause of the "minor" incident was that a nut was not properly tightened during maintenance by Boeing workers, said Megumi Tezuka of All Nippon Airways.
A Boeing spokeswoman said: "We are aware of the event, which was resolved quickly as noted by ANA."
Both companies said the issue did not involve overheating lithium batteries, an issue that prompted the Federal Aviation Administration to ground all Dreamliners in January.
Boeing has been working to retrofit the 50 Dreamliners it delivered before the grounding with a battery fix since January.
ANA, the largest customer for the Dreamliner with 17 of the planes already delivered, announced this month it plans to return its fleet of Dreamliners to service on June 1.
CNN's Dave Alsup contributed to this report.