A Supreme Court case over a $15,000 tax bill could have multibillion-dollar ramifications. The court must rule narrowly — because if it doesn’t, much of our tax code could enter constitutional purgatory.

In December, justices will hear oral arguments in Moore v. United States, a landmark tax case to decide the legality of taxing unrealized income. The stakes are high: If the court sides with the plaintiffs, it could inject great uncertainty into our already complex tax code, result in billions (or even trillions) of lost revenue, and invite mountains of new litigation. But if it sides with the lower court’s ruling, it could open the door for wealth taxes, widely regarded as both economically harmful and unconstitutional.

The saga begins with a Washington state couple and the Tax Cuts and Jobs Act of 2017.

Charles and Kathleen Moore owned 13% of a small company in India that made farm equipment. Each year, the company generated profits but reinvested them in the business rather than distributing them to the Moores and other investors. Before 2017, U.S. citizens were required to pay taxes on their overseas profits but could defer those payments if they kept the money abroad. This created a “lockout effect,” incentivizing keeping profits in foreign countries.

The TCJA changed this. It moved the country toward a “territorial” tax system, only taxing profits earned in the U.S. It ended deferral and created a one-time Mandatory Repatriation Tax at a low rate on income held abroad and all the deferred taxes coming back into the U.S.

As a result, the Moores were hit with a $14,729 tax bill, even though they hadn’t sold their stock in the company or received a dividend. ( Recent reporting , however, suggests the Moores did receive compensation from the company, and they may have simply received bad tax advice.)

The question before the court: Can Congress tax this income given the 16th Amendment and subsequent case law generally requiring that income be “clearly realized” before it is taxed?

The court’s answer could have widespread implications. On one extreme, it could endorse a new definition of income, paving the way for taxes that target unrealized assets — a policy that inhibits the very financial growth it plans to tax. Conversely, other scenarios could render large portions of the U.S. tax base (far beyond the provision contested by the Moores) legally uncertain, putting significant revenue at stake.

If the Supreme Court strikes down all of the MRT for corporate and noncorporate taxpayers, the Tax Foundation estimates this could reduce revenue by about $346 billion over the next 10 years. A similar, but more narrow, ruling only striking down the MRT for pass-through firms (businesses that report their incomes on individual tax returns) and people could reduce overall revenue collections by about $3.5 billion over the next 10 years.

A broader ruling, that actual (rather than deemed) repatriation is a requirement for income to face U.S. tax, may implicate other major corporate tax policies, including the new 15% corporate alternative minimum tax on book income, the tax on global intangible low-taxed income established in 2017, and the Subpart F rules that date back to 1962. All told, if these three policies are struck down, then $667.9 billion in federal revenue over the next 10 years would be at risk.

The most extreme outcome would be eliminating taxes on all undistributed business earnings, regardless of where they’re earned. Though unlikely, this would put nearly $5.7 trillion in federal revenue over 10 years at risk.

The court must walk a fine line between endorsing wealth taxes on the one hand and increasing uncertainty in the tax code on the other.

A broad decision could hamstring Congress’s ability to write tax rules. And while wealth taxes are economically unsound tax policies, the court doesn’t really have a wealth tax case in front of it, as the income was earned and realized for use offshore.

So, what’s the court to do?

Rule very narrowly. If the justices issue a broad ruling — by, say, striking down the MRT or invalidating the TCJA — then they could tie Congress’s hands when trying to write rules defining what constitutes income. But there could be a narrow path to forestall true wealth taxes while letting Congress retain its ability to legislate.

Much of the complexity in our tax code is due to this challenge of defining “income.” But the proper venue to settle the debate is Congress, not the Supreme Court. With many income tax provisions set to expire in the next few years, the court should give Congress the opportunity to resolve the matter rather than putting long-standing tax rules into uncharted waters.

Daniel Bunn is president and CEO of the Tax Foundation, a nonpartisan tax research organization in Washington, D.C.