Dozens of subway riders in New York City are being charged extra for their transit rides, and the Metropolitan Transportation Authority suspects that Apple may be partly to blame.

The MTA is not explicitly pointing the finger at Apple, but it appears that some riders may be unwittingly putting their iPhones too close to the system’s new tap-to-pay OMNY readers while simultaneously swiping their MetroCards at the turnstile.

Last spring, the MTA began installing tap-to-pay readers as part of its new fare payment system called OMNY, which stands for “One Metro New York.” Soon after, Apple announced a new feature called Express Transit, which would allow customers to pay for riding the subway without opening an app or unlocking their phone. They simply needed to place their iPhones on the reader and then walk through the turnstile.

The trouble is some people have the Express Transit feature enabled, but are still using their MetroCard to pay for rides. While swiping their MetroCard, their iPhones are somehow coming into contact with the reader, either because they were holding it in their hands or through a pocket or purse, the MTA suspects. However it happens, they are getting charged twice for the ride. Gothamist, which first reported the incidents, highlighted the tweets from some of the people who were doubly charged.

Naturally, many of these people assume the problem is with the MTA, and not their own behavior. But the MTA insists that it is not to blame, and that it’s working with Apple to address the problem.

“We’ve been notified by about 30 customers, out of more than 5,000,000 taps so far, about unintended charges when the Express Transit feature of their iPhones is activated,” Al Putre, the head of OMNY at the MTA, said in a statement. “The system is a popular success that’s working well for the vast majority of people and we’re in touch with Apple about addressing the issue of unintended taps.”

Express Transit first arrived with iOS 12.3 and watchOS 5.2.1 updates. Apple billed it as an easy and convenient way to pay for transit with Apple Pay without having to use Touch or Face ID to unlock your phone. The MTA hopes to have OMNY readers installed in every subway station and on all bus routes by late 2020, at which time they’ll add more fare options.

Initially, Apple customers had to enable Express Transit through the settings on their iPhones. People who live in New York City who add a credit or debit card to their iPhones are asked whether they want to enable Express Transit. Some of these people probably said yes without realizing what the feature did, and then continued to pay for subway rides with their MetroCard.

In addition to New York, Apple’s Express Transit is available to transit riders in Portland, London, Shanghai, Beijing, and Tokyo.