President Donald Trump indicated in an interview with Fox News Friday that he would invoke executive privilege to prevent former national security adviser John Bolton from testifying at the Senate impeachment trial. Earlier this week, Bolton said in a statement that he is “prepared to testify” if the Senate issues a subpoena for his testimony.

In the interview, Laura Ingraham asked Trump why he would not allow Bolton to testify if he knew that his former adviser had information that would help the president make the case that he had done nothing wrong. Trump said he would have “no problem” with Bolton testifying—“other than one thing.” Trump insisted he was watching out for his successors. “You can’t be in the White House as president, future, I’m talking about future—many future presidents—and have a security adviser, anybody having to do with security, and legal and other things but especially…,” Trump said before he was interrupted by Ingraham, who asked whether he would invoke executive privilege to block the testimony.

“I think you have to,” Trump said. “For the sake of the office.” The president went on to say that he would “love everybody to testify,” listing White House chief of staff Mick Mulvaney, Secretary of State Mike Pompeo and former Energy Secretary Rick Perry as examples. But doing so, would hurt presidents in the future, Trump insisted. “There are things that you can’t do from the standpoint of executive privilege,” he said. And Trump went on to make clear that Bolton’s testimony would be particularly harmful. “Especially, a national security adviser,” Trump said. “You can’t have him explaining all of your statements about national security concerning Russia, China, and North Korea—everything. We just can’t do that.”