Wells Fargo & Co customers aiming to sue the bank over bogus accounts opened in their names may be in for an unpleasant surprise: the fine print requires them to take their claims to an arbitrator instead of a court.

Mandatory arbitration rules inserted into account-opening agreements prohibit customers from joining class actions or suing the third-largest U.S. bank in court. Instead, the agreements require individual, closed-door arbitration.

U.S. senators highlighted the issue on Tuesday as they grilled Wells Fargo Chief Executive Officer John Stumpf during a hearing.

Asked if he would set aside the mandatory arbitration agreements for customers affected by the phantom accounts, Stumpf demurred.

“I’m not an expert in that,” he said, adding he would talk to his legal team.

That was not enough for some lawmakers.

“If we had class action on this in 2010, 2009, 2008, the problem never would have gotten so out of hand,” Senator Elizabeth Warren, a Democrat from Massachusetts, said later, when questioning regulators about the practice.

Class actions can be more affordable for unhappy customers, especially those with limited resources, because they can band together to sue, rather than having to hire lawyers individually. Consumers also complain that target companies often choose the arbitrators; proceedings are confidential; and decisions are hard to appeal.

Three Wells Fargo customers filed a lawsuit Friday in a Utah federal court, seeking class action status on behalf of hundreds of thousands of customers nationwide they say were harmed by the San Francisco-based bank’s fraud and recklessness.

It was unclear whether they could get around the mandatory arbitration clauses, though. Last year, Wells successfully invoked the clauses to defend against a class action suit tied to bogus accounts.

In that case, the judge said customers had to arbitrate because of agreements they signed when opening legitimate accounts at the bank.

The Consumer Financial Protection Bureau, a brainchild of Warren, was part of the regulatory group that negotiated a $190 million settlement from Wells Fargo over the bogus accounts.

The bureau is considering rules to ban banks, credit card issuers and other companies from forcing customers to submit to arbitration and waive their right to join class action lawsuits.

Under the proposal, companies could still use arbitration, but would have to tell consumers they could join class action lawsuits instead.

Mandating arbitration when signing up for financial products has become standard practice after a 2011 U.S. Supreme Court decision validated the practice.

Still, the tide may be turning, said Joseph Peiffer, a New Orleans lawyer who has represented investors and others in class action lawsuits.

“Class actions dissuade companies from ripping people off a thousand dollars here and a thousand dollars there,” Peiffer said.

(Reporting by Suzanne Barlyn. Editing by Carmel Crimmins and Jonathan Stempel; Additional reporting by Lisa Lambert and Dena Aubin; Editing by David Gregorio)