Waymo is recalling 3,791 autonomous taxis after a software defect caused some vehicles to slow and then drive into flooded roadways on higher-speed roads, the company said in a filing made April 30. The voluntary recall affects vehicles running on Waymo’s fifth and sixth generation Automated Driving System.

The recall came after severe weather in San Antonio, where Waymo said one vehicle entered a flooded and impassable road. The company said all affected vehicles have already received an interim software update while a full remedy is still being developed.

Waymo said the problem centered on a gap in how its vehicles handled untraversable flooded lanes on faster roads. A company spokesperson said the firm had identified “an area of improvement regarding untraversable flooded lanes specific to higher-speed roadways, and have made the decision to file a voluntary software recall with NHTSA related to this scenario.”

The National Highway Traffic Safety Administration said entering a flooded roadway can cause a loss of vehicle control and increase the risk of a crash or injury. Waymo said it has increased weather-related constraints on its vehicles and is working on additional software safeguards to keep them out of standing water.

The recall is the latest in a series of safety-related issues for the robotaxi operator, which last year recalled more than 1,200 autonomous vehicles after minor crashes involving obstacles in the road. Earlier this year, one of its vehicles struck a child outside a school in Santa Monica, and another fatally ran over a neighborhood cat in San Francisco.

Waymo operates in 10 major cities and says it provides over half a million trips every week. The company says data from more than 170 million fully autonomous miles shows its system is 13 times safer than human drivers in crashes involving pedestrians, but the recall underscores how even the most advanced self-driving car systems can fail in conditions that are ordinary for human drivers and difficult for software to judge.

Waymo is ahead in the race to scale robotaxis across the country, with thousands of vehicles carrying paying customers in cities including Los Angeles, Miami and Phoenix. The open question is how quickly it can finish the full remedy and prove that a fleet built to avoid danger can reliably recognize the kind of flooded road that a person would never enter.