Waymo is recalling its autonomous driving software that allowed its vehicles to drive on flooded roads. The recall affects 3,791 vehicles that operate using Waymo’s fifth and sixth generation systems.

In documents filed with the National Highway Traffic Safety Administration, the Alphabet-owned company said that an unoccupied Waymo robotaxi “encountered an untraversable flooded section of a roadway that has a 40 mph speed limit.” Despite detecting the flooded road, the vehicle “proceeded at reduced speed.” Waymo said it was currently working on a remedy. In the interim, the company has updated its vehicles to “increase weather-related constraints and updated the vehicles maps.”

While no one was injured in the incident, it highlights the risk for driverless car companies when encountering altered road conditions as a result of extreme weather. In its first few years of operation, Waymo has strategically stuck to cities with warmer, drier climates — places like Phoenix, Los Angeles, Atlanta, and Austin. But as it eyes a slate of East Coast cities, including Boston, New York City, and Washington, DC, for the next phase of its expansion, its abilities to handle more adverse weather will become a crucial test.