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On Thursday—about a month after author Michael Lewis published a book charging that the stock market is “rigged”—the Securities and Exchange Commission, a Wall Street regulator, slapped a $4.5 million penalty on the New York Stock Exchange and several affiliated companies for “business practices that either violated” their own rules “or required a rule when the exchanges had none in effect.”

Some of the SEC’s complaints related to the high-speed, computerized trading practices covered in Lewis’s book. For example, the agency said that the exchange didn’t have rules to ensure that colocation is “fair and equitable.” Colocation is exchanges’ practice of charging brokers for the privilege of putting their computers close to the exchanges’ machines, which cuts down on the amount of time it takes to transmit orders.

Lewis’ book, Flash Boys, also takes on high-frequency trading—when computers running complex algorithms are able to trade faster than humans, and thus turn a profit for investors from an accumulation of split-second advantages. (My colleague Nick Baumann has more about high-frequency trading here.) Experts say Thursday’s action by the SEC could fuel the debate about whether certain firms gain an unfair trading edge by using this type of technology.

The SEC stopped short of calling the NYSE’s violations illegal, and the exchange agreed to pay the fine—which is small in comparison to its earnings—without admitting wrongdoing.