Hong Kong (CNN Business) An alert system that was supposed to be a standard feature on Boeing's 737 Max aircraft "was not operable on all airplanes," the company has said.

Boeing said in a statement Monday that the function wasn't working on some of its planes because it was mistakenly linked to an optional feature, the angle of attack (AOA) indicator.

"The disagree alert was intended to be a standard, standalone feature on MAX airplanes. However, the disagree alert was not operable on all airplanes because the feature was not activated as intended," it said. "Unless an airline opted for the angle of attack indicator, the disagree alert was not operable."

The AOA indicator lets pilots know whether one of the AOA sensors is not working. The "disagree alert" shows if the sensors contradict each other. Boeing said the alert function was not necessary for the safe operation of the airplane.

A Boeing 737 Max 8 airliner lifts off for its first flight in 2016 in Renton, Washington.