A subsidiary of Vitol Group, the biggest independent oil trader, is demanding that the U.S. government return more than $52 million in fuel taxes the Rotterdam-based company says was improperly collected.

The Internal Revenue Service failed to refund federal excise taxes paid by Vitol Inc., which is principally operated out of Houston, from 2014 through 2017, the company said Tuesday in a filing in federal court in Houston.

Vitol, which handles about 7.4 million barrels of crude and petroleum products per day, alleges the U.S. failed to classify fuel blends that included liquefied petroleum gas, butane and gasoline as “alternative fuel mixtures” eligible for tax refunds during the period.

The company, one of the biggest exporters of U.S. crude oil, says its production of alternative fuel mixtures entitled it to tax credits.

The IRS “commissioner has improperly and unlawfully failed to refund” excise taxes paid, Vitol said in the suit.

IRS spokeswoman Sarah Maxwell declined to comment, citing the agency’s policy on pending litigation.

Founded more than a half century ago, Vitol has grown from a small fuels merchant in Europe to a dominant energy trader with offices in London, Geneva, Houston and Singapore. Its U.S. trading operations are its largest, growing rapidly in recent years as soaring production of American shale oil and gas has upended global petroleum markets.

A Vitol spokeswoman in London declined to immediately comment on the lawsuit.