We’re sorry, sir, but we don’t have any cars left.

That was my unpleasant welcome to Michigan by Hertz.

I had a reservation. They saw the reservation. The problem: Hertz hadn’t actually saved me a car.

Reserving a car is different than almost any other travel product.

Airfare is typically non-refundable once you purchase a ticket. Hotel rooms can be canceled up to a certain point. But there’s typically no penalty for reserving a car and never picking it up.

That leaves the industry with many more reservations than actual renters.

So just like airlines sell more tickets on planes than seats, car rental agencies sometimes don’t have enough cars to meet their demand. Paula Rivera, a spokeswoman for Hertz Global Holdings Inc., parent company of Hertz, Dollar and Thrifty, said via email that the situation occurs about 1 percent of the time.

If there are no cars left at the airport, Hertz will let customers rent from a competitor and pay the difference, or pay for a cab to and from your hotel, asking you to return the next morning when more cars might be available.

The company will also provide a $50 voucher for a future rental.

Alice Pereira, a spokeswoman for Avis Budget Group Inc., outlined a nearly identical policy at her company. A representative for Enterprise Holdings, the parent of Alamo Rent A Car, Enterprise Rent-A-Car and National Car Rental, did not respond to requests for comment.

Hertz’s Rivera said that travelers whose plans are concrete can prepay for their rental, “which helps to ensure availability upon arrival.”

It also pays to sign up for the rental firms’ loyalty program, which is free. Rivera said members of Hertz’s Gold Plus Rewards program, “by virtue of their loyalty, are served first.”