It looks like proprietary trading. It walks and talks like proprietary trading. A Goldman Sachs junk-bond trader earned more than $100 million in profits buying and selling distressed corporate debt earlier this year, but according to a Wall Street Journal report on Wednesday, the trades don’t violate the so-called Volcker Rule because the trader is a market-maker not a soon-to-be prohibited proprietary trader.

The Volcker Rule isn’t yet in effect. In July, the Federal Reserve said it would extend the deadline for banks to comply with the Volcker Rule until the middle of 2017, seven years after the Dodd-Frank law was first passed.

That said, Goldman Sachs GS, -1.22% has publicly said it’s begun implementing the rule, and it plans “to continue to conduct our investing and lending activities in ways that are permissible under the Volcker Rule.”

The main distinction in the Dodd-Frank Act’s Volcker Rule is whether the trader intended to serve the needs of clients for liquidity, or was not interested in that, said Darrell Duffie, a professor of finance at Stanford University Graduate School of Business.

“How would we know?” Duffie asked.

It’s a distinction without an easily discernible difference. The Volcker Rule, one of the most hotly contested components of the post-crisis banking reform law passed in 2010, bans proprietary trading by banks and their affiliates. However, it exempts proprietary trading that is related to market-making, among other exemptions.

Stanford University Graduate School of Business finance professor Anat Admati, a vocal critic of the big banks, told MarketWatch that the market-making exemption is supposed to be primarily customer focused. “It’s implausible to make such outsized profits on what is essentially a customer accommodation.”

In a report to the Securities Industry and Financial Markets Association in 2012 that was provided to financial regulators to help with implementation of the Volcker Rule, Duffie tried to make the distinction. “Market making is proprietary trading that is designed to provide ‘immediacy’ to investors,” wrote Duffie. “Proprietary trading is the purchase and sale of financial instruments with the intent to profit from the difference between the purchase price and the sale price.”

The Wall Street Journal report says that the trader, Tom Malafronte, started to accumulate the junk bonds beginning in January 2016. By the end of Goldman Sachs’ second quarter, Malafronte had booked the profits.

The Wall Street Journal article says Malafronte “bought the bonds from clients anxious to sell them and ultimately lined up other investors to buy them—at a higher price.” He started accumulating his positions in January, and held some bonds for weeks and in other cases for only a very short time, reselling them sometimes within 24 hours. In all cases he seems to have put Goldman’s capital at risk.

A spokeswoman for Goldman Sachs didn’t respond to a request for comment.

A Volcker Rule study completed by the Financial Stability Oversight Council anticipated the challenges in implementing the Volcker Rule as written.

The Dodd-Frank law says “market-making-related” activity is a permitted activity, as long as it is “designed not to exceed the reasonably expected near term demands of clients, customers or counterparties.” The challenge is that market-making businesses often include elements of proprietary trading. “Agencies,” the report says, “therefore must be vigilant to ensure that banking entities do not conceal impermissible proprietary trading activities within larger market making operations.”

The FSOC report seems to place the Goldman Sachs trading closer to proprietary trading rather than the exempted market-making especially because it involves a less liquid market like “junk” corporate debt. According to Goldman Sachs’ filings, its corporate bond portfolio is recorded at fair market value, with gains and losses on the bonds required to be recognized each period. However as of December 31, 2015, 12.5% of the market value of its corporate debt held was priced according to a model, with significant estimates and judgments used to assign a value for the balance sheet each period rather than easily verified quoted prices.

Market-making becomes quite “complex”, the FSOC report says, “in illiquid markets or in a liquid market where an order is very large, as a market maker may be required to assume significant market risk between the time that the large order is purchased and sold back into the market.”