This article has been updated to reflect comment from Tesla.
In a test simulating an off-set frontal collision at 40 mph, the insurance Institute for Highway Safety (IIHS) announced Wednesday that the Model S earned an "acceptable" rating, which is the second-best designation, Business Insider reports. The IIHS is a research affiliate within the insurance industry.
The reasoning, according to Business Insider, is a seat belt issue.
During the crash test, the Model S's seat belt didn't stop the driver's head from slamming into the steering wheel through the deployed air bag — thus resulting in a second-best rating for the vehicle.
In a statement to Fortune, however, a Tesla spokesperson pushed back on the findings, arguing hat the Model S should have been given a higher rating.
"While IIHS and dozens of other private industry groups around the world have methods and motivations that suit their own subjective purposes, the most objective and accurate independent testing of vehicle safety is currently done by the U.S. Government which found Model S and Model X to be the two cars with the lowest probability of injury of any cars that it has ever tested, making them the safest cars in history," the spokesperson said.