Seven Philadelphia residents have challenged the constitutionality of Philadelphia’s wage tax, which levies residents’ incomes earned outside of Pennsylvania. And, according to a recent essay in the University of Pennsylvania Law Review, the Constitution is on the taxpayers’ side.

The essay, written by Penn Law professor Michael Knoll and University of Virginia Law professor Ruth Mason, argues that Philadelphia’s wage tax on residents’ out-of-state income violates the Constitution’s Commerce Clause, which gives Congress exclusive authority to regulate interstate commerce. That means states—and municipalities like Philadelphia by extension—cannot put burdens, like tariffs, on commercial activity that crosses borders.

The law review article cites a December PlanPhilly article that first reported the similarities between Philadelphia’s wage tax and a similar tax scheme in Maryland that the Supreme Court of the United States declared unconstitutional in a May 2015 decision.

When a Philadelphia resident pays income taxes to Philly for work done outside of the city, and also pays taxes on the same income to another state or municipality, they incur a double taxation. Without some sort of credit or other offset, that double taxation is an unconstitutional burden on interstate employment.

Officials in Maryland cured their income tax’s constitutional deficiencies by offering eligible taxpayers refunds. Philadelphia is following their example, says Mike Dunn, a spokesman for Mayor Jim Kenney: “We do know that some Wage Taxpayers are eligible for refunds.”

Indeed, two refund appeals have been granted, for a total of $7,000. Another appeal has been denied, and four more are still pending. Dunn said the seven petitions requested a grand total of $100,000 in wage tax refunds.

Not every city resident who works outside of Pennsylvania will be eligible for refunds. Pennsylvania has income tax reciprocity agreements with New Jersey, Indiana, Maryland, Ohio, Virginia and West Virginia, meaning that residents of those states only pay income taxes in their home states, and not the states they work in.

The potential impact on Philadelphia’s budget still isn’t clear to city officials. “The Administration continues to evaluate the Supreme Court decision. We do not currently have an estimate of its impact, as the facts of the issue continue to evolve,” said Dunn in an emailed statement.

Potentially eligible taxpayers can begin the refund process by filing a refund request form, available on the City’s website (non-salaried employees should use this form), but, given the legal complexity, taxpayers may want to seek legal representation before submitting any refund requests.

In Delaware, Wilmington’s income tax, nearly identical to Philadelphia’s, faces similar constitutionality questions.