Upset by a tell-all book, President Donald Trump has again threatened a former staffer by noting that they signed a non-disclosure agreement, a move that historians say is unprecedented.

When former campaign aide and White House communications adviser Cliff Sims’ book “Team of Vipers” came out this week, Trump tweeted that his former staffer was a “mess” and “nothing more than a gofer,” adding “he signed a non-disclosure agreement.”

Lawyers for Trump’s campaign reportedly filed an arbitration claim against Sims on Thursday, claiming he violated the agreement by criticizing Trump and revealing private conversations in the book.

It was only the latest in a series of times when Trump has alluded to non-disclosure agreements signed by his former campaign aides and staffers or had his lawyers take legal action. Historians say that previous presidents did not threaten to enforce NDAs against former West Wing staffers, who technically work for the U.S. government. It’s unclear if White House NDAs are even legally enforceable.

“As far as I know, it’s never been tried in the White House,” said Russell Riley, the co-chair of the Presidential Oral History Program at the University of Virginia’s Miller Center. “I’ve never heard if it. I’ve done oral histories back to the Jimmy Carter presidency.”

All presidents want to keep their policy deliberations as closely held as they can, and former staffers spilling secrets have always been a concern. But officials with security clearances already are barred by law from releasing classified information, and most staffers are either personally loyal to the president or are careful about what they share to protect their career futures.

But Trump hasn’t just targeted Sims.

When former “Apprentice” star and White House staffer Omarosa Manigault Newman released audio recordings of conversations with Trump in the White House in advance of the publication of her book “Unhinged: An Insider Account of the Trump White House,” Trump wrote on Twitter that Manigault Newman had signed a non-disclosure agreement.

Lawyers for Trump also reportedly sent a letter to Steve Bannon last year saying Bannon had violated an agreement not to disparage Trump by talking to author Michael Wolff for Wolff’s book “Fire and Fury: Inside the White House.”

The limits of such agreements are already being tested in court. In August, a New York judge ruled that nondisclosure agreement signed by a former campaign staffer named Jessica Denson could not force into private arbitration her lawsuit alleging sexual discrimination and harassment while working on the Trump campaign.

Riley said the threats show that Trump does not have the same level of trust and professionalism with his staff that his predecessor had.

“If you properly vet people, there is an expectation that you would only bring people into the White House who would use their best judgement. It would never occur to sign an NDA to do what they should be doing out of their own conscience and professionalism,” he said.

Apart from that, he argued that it strikes against the idea that White House staffers are public servants.

“These people are not working for Donald Trump, they are working for the citizens of the United States of America,” he said.

Contact us at letters@time.com.