Former Treasury Secretary Lawrence Summers says the thought of a Donald Trump presidency “scares me.”

“I don’t know of a greater threat to our prosperity,” Summers told Fox Business Network’s Wall Street Week. “I think policy would be extremely erratic, I think there are great risks to the global economy from the truculence that would manifest,” he said when asked about Trump.

Summers is also concerned about Trump’s influence on the fiscal reputation of the United States, FBN reported.

“I think there are great risks even going to the government’s credit domestically. I think it would put an uncertainty premium the likes of which we haven’t seen before into the markets,” said Summers, now a Harvard professor.

Summers, who also was an economic adviser to President Barack Obama and served as Treasury secretary under Bill Clinton, counted the rise of political populism in the U.S. among the biggest economic risks at the SkyBride Alternatives Conference in Las Vegas, Fortune said.

“Political risk driving huge economic risk is something I always thought you talked about in connection with emerging markets. Now I think it is something you talk about in respect to the United States,” he said. “And the kind of risk of high populism taking over and that leading to huge instability is a risk now in the United States.”

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