China has worked to undermine U.S. tariffs on solar equipment and is “exploiting” a loophole that shields some panels from the duties, according to White House trade adviser Peter Navarro.

The country has tried to skirt the tariffs that the U.S. government imposed on solar imports last year, Peter Navarro, an assistant to U.S. President Donald Trump on trade and manufacturing policy, said by email Wednesday. More specifically, he said, China is taking advantage of an exemption for bifacial—or double-sided—solar panels, adding that it’s a loophole that must be “slammed shut.”

The Trump administration took the renewable energy industry by surprise in June when it cleared an exemption for bifacial solar panels, along with a few other products. The carve-out was widely seen as offering an escape from the tariffs since U.S. utilities were already slowly transitioning to using the double-sided panels. Less than four months after granting the reprieve, however, the government decided to yank it, saying it would’ve led to more imports.

Navarro’s comments came as the U.S. Court of International Trade considers an appeal of the revocation. The court delayed the government’s withdrawal of the carve-out until at least Thursday.

“It is unfortunate that China continues to attack America’s solar industry with its unfair trade practices and attempted end runs around the tariff,” Navarro said.