Robert Gates thinks that Hillary Clinton’s email scandal “is really a concern.”

The former secretary of defense was asked about the temperaments of Donald Trump and Clinton and which one would be better fit to be president, in an interview with Yahoo News on Wednesday,

Gates — who worked in both the Bush and Obama administrations and worked closely with Clinton — declined to weigh in. He said that while he had worked closely with the former secretary of state he didn’t know what Trump was like beyond his public persona.

But when pressed further on what he thought about Clinton, Gates brought up the investigation over her private email server.

“There’s the whole email thing, which I think is really a concern in terms of judgement,” Gates said. “I don’t know what originally prompted her to think that was a good idea.”

“Using an offline server I think was an error,” he added.

Gates also discussed Turmp’s foreign policy stance.

“Mr. Trump’s comments have been essentially, I think, ill-informed bluster and threats,” he said.

“Would you feel comfortable with his proverbial finger on the nuclear button?” host Katie Couric asked.

“Right now, no. But the question is does he moderate his views on national security issues going forward?” Gates said. He added that Trump would need to get more informed views on some of the challenges the U.S. faces and surround himself with good advisers.

But that doesn’t mean he’s going for Clinton either.

“I haven’t heard much on foreign policy, national security policy that gives me comfort out of either set of candidates,” he said.