The Shark Tank star mocked the former president’s proposal to raise tariffs, which Trump has said could reach 10% or 20% for all products coming into the U.S., and added that it would amount to “a tax on everybody.”

Cuban said he believes Trump, at least in the 1990s and early 2000s, “used to know how tariffs work,” but those days are long gone. 

“I don’t know what happened to him,” Cuban said.

Cuban also criticized Trump’s proposal to enact a 60% tariff on imports from China, which the former principal owner of the Dallas Mavericks said was not a good idea.

“This man has so little understanding of tariffs he thinks that China pays for them,” Cuban said. “This is the same guy who also thought that Mexico would pay for the wall.”

Cuban explained that Americans don’t realize how many of their everyday products are made in China, and with a proposed tariff on Chinese imports, the price of those products could skyrocket by 50% or more by Christmastime next year. He added that local business owners could also feel the pain of tariffs, and even be forced to close up shop.

The Trump campaign did not immediately respond to Fortune‘s request for comment.

Economists have said Trump’s tariff proposals would raise costs for middle-class families. The tariffs, if implemented, could cost the average American household more than $2,600 a year, according to an August study by the Peterson Institute for International Economics. 

Trump has previously mischaracterized how tariffs work, calling them “a tax on other countries.” In reality, tariffs are paid by American consumers buying imported products. The cost of a 10% tariff (as Trump has proposed) could reach $300 billion per year for American consumers, wrote Erica York, a senior economist at the right-leaning Tax Foundation.

Trump has said he sees tariffs as a way to raise money so the government can lower taxes, and his former top trade official, Robert Lighthizer, has said tariffs should be aimed at eliminating the U.S. trade deficit. Yet Trump’s economic plans have the potential to reshape global trade, and hiking tariffs on other countries could lead them to enact or increase tariffs on U.S.-made goods. 

For Cuban, this move away from free-trade policies is a step in the wrong direction. 

“The way [Trump has] been thinking about tariffs and trade now, something’s a little bit lost,” he said. “And now his trade policies, particularly with tariffs, are basically just gibberish.”