An issue with self-driving cars that apparently no one previously considered has come to light: dozing passengers.

Officials in San Francisco, where Alphabet’s Waymo company and GM-backed Cruise are currently operating robotaxi services as part of ongoing trials, highlighted the problem in a recent letter to the regulator, Wired reported.

Signed by the San Francisco Municipal Transportation Agency, the San Francisco County Transportation Authority, and the Mayor’s Office on Disability, the letter to the California Public Utilities Commission (CPUC) explained that in the last couple of months alone, there have been three incidents where Cruise staff were compelled to call 911 after a rider in one of its driverless vehicles failed to respond to their calls via the two-way voice link inside the vehicle.

When police and firefighters arrived on the scene, they found nothing more than a dozing passenger enjoying — apparently rather too much — the comforts of a driverless ride.