A Waymo took the plunge, and now its human developers are paying for it: the autonomous driving company says it’s recalling 3,791 of its robotaxis after one of them decided to test its seaworthiness and plow into floodwater.

Reflecting a broader issue with its fleet, Waymo’s cars when operating on higher speed roadways “may slow but not stop in response to detecting a potentially untraversable flooded lane,” the Google-owned company conceded in a notice filed with the National Highway Traffic Safety Administration last week.

It sounds like it may not be a simple fix. Until it develops a “final remedy,” Waymo said it’d limit its cars’ access to higher-speed roadways that are vulnerable to flash flooding.

“We are working to implement additional software safeguards and have put mitigations in place, including refining our extreme weather operations during periods of intense rain,” the company said in a statement, per Bloomberg.

The recall stems from an April 20 incident in San Antonio, Texas, when a Waymo encountered a flooded section of roadway with a 40 mph speed limit. The car should’ve stopped or avoided the floodwater, but it instead drove straight into it at a “reduced speed,” per the recall notice.  

According to local reporting, the Waymo was swept away by the floodwater and dumped in the Salado Creek, and wasn’t recovered until four days later. Narrowly avoiding disaster, there was no one riding in the amphibian manqué at the time of its ill-fated voyage.

This isn’t Waymo’s first floodwater-related gaffe in San Antonio, though it’s certainly its most dramatic. Weeks before, another Waymo had to be towed after getting stuck in a mildly flooded crossing. It didn’t appear to be at risk of being swept away, however. 

Water, it appears, is the robotaxis’ kryptonite. Recent footage showed a Waymo stop in the middle of a busy roadway in Austin, Texas because of a shallow puddle, clogging the traffic behind it.

The incidents will raise serious doubts over autonomous vehicles’ ability to handle hazardous, or merely less than ideal, road conditions. Tesla’s Full-Self Driving mode struggles to navigate in fog and even direct sunlight, and its own Robotaxi service drew alarm after footage showed one of its cabs blasting down partially flooded Austin streets at high speeds. And the Waymo fleet in San Francisco descended into chaos when a power outage left the streets without working traffic lights last December.

More on self-driving: China Presses Pause on Self-Driving Taxis Nationwide After Issue Where They Blocked Streets