The U.S. Supreme Court struggled during arguments Tuesday with how to determine when federal law trumps state laws regulating national banks, with Justice Samuel Alito Jr. worried about the “practical nightmares” that could come from the court’s decision.

In Cantero v. Bank of America, the justices are reviewing the U.S. Court of Appeals for the Second Circuit’s decision that the 1864 National Bank Act preempts a 1974 New York law requiring national banks to pay at least 2% interest payments on mortgage-escrow accounts.