Hillary Clinton reiterated her call for Donald Trump to release his tax returns, suggesting that they would reveal he doesn’t pay federal income tax.

“If you’ve got someone running for president who’s afraid to release his tax returns, because it will expose the fact that he pays no federal income tax, I think that’s a big problem,” the former secretary of state said Sunday on NBC’s “Meet the Press.”

“The only two [tax returns] we have show that he hasn’t paid a penny in taxes,” Clinton continued, referring to a Washington Post report that found that for at least two years in the 1970s, Trump took advantage of a tax loophole allowing him to report negative income and pay no income tax.

Trump has previously suggested he can’t release his tax returns because he is undergoing an audit by the Internal Revenue Service. The IRS later clarified that there is no prohibition on individuals sharing tax information while being audited. 

Earlier this month, George Stephanopoulos pressed Trump to disclose his effective tax rate. “It’s none of your business,” Trump responded. “You’ll see it when I release, but I fight very hard to pay as little tax as possible.”

If Trump fails to release his tax returns before the election, he would be the first major-party nominee in several decades to do so.

Senate Majority Leader Mitch McConnell (R-Ky.), who has subtly signaled his distaste for the presumptive Republican nominee, pointed to this precedent last week when asked if Trump should release his tax returns.

“He’ll have to make that decision himself, but that’s certainly been the pattern for quite some time,” McConnell said.