(Reuters) - Amazon.com's ( AMZN ) self-driving unit, Zoox, has recalled 258 vehicles due to issues with its automated driving system (ADS) that could cause unexpected hard braking, the company said on Wednesday.

The recall affects vehicles equipped with ADS software versions released before November 5, according to a report Zoox filed with the National Highway Traffic Safety Administration last week.

The California-based company said it has addressed the issue by updating the ADS software on affected vehicles.

While the company has not publicly disclosed the number of vehicles on the road, it has ramped up testing over the past year.

Last June, the company announced plans to begin testing its autonomous vehicles in two new cities.

Self-driving vehicle companies, including General Motors' Cruise and Google-owned Waymo, are under heightened scrutiny following a 2023 incident where a pedestrian was seriously injured by a Cruise vehicle.

Last year, Waymo recalled more than 670 self-driving vehicles after one of its driverless vehicles struck a wooden utility pole in Phoenix, Arizona.