Sebastian Thrun is an Uber driver whose car is a Tesla and who got five-star ratings from users who booked him. He’s also an inventor of Google’s self-driving car, cofounder of the Google X innovation unit, and cofounder and CEO of the Udacity online-learning company.

Thrun says he was recently blocked by Uber from picking up any more passengers, and he doesn’t have any explanation for why.

“I’m pretty sure I was put on hold by a computer program,” Thrun says. The only possible reason he can think of is that a few friends booked him via Uber’s app when they saw him in the service and then canceled the rides.

“If you write about it, maybe you can say, ‘Please unblock Sebastian,’” says Thrun. He holds up the screen of his phone to display proof of his five-star-average rating.

Uber declined to comment.

Thrun, who also is a Stanford University research professor, says he started moonlighting as an Uber driver last month hoping to better understand the experience of workers in the on-demand economy. He completed two trips in the vicinities of Palo Alto and San Jose, California, and made $52.61 after Uber deducted its fees. “I have to admit—I really enjoyed being an Uber driver,” Thrun says.

Before starting, Thrun submitted to a background check. He was required to take his 2013 Tesla in for an Uber inspection, where he said he had to convince the inspectors that the electric car’s digital parking brake complied with safety regulations.

On his first trip, he wound up driving a Stanford student who had taken Thrun’s class on artificial intelligence and recognized him partway through. Thrun’s second passenger was a corporate executive who tried to tip him at the end of the ride. Thrun refused.

“Money is the last thing I need,” he said.