Former White House communications aide Cliff Sims, who made headlines with his tell-all book Team of Vipers, is suing President Donald Trump, claiming the president uses his campaign organization to punish former employees for exercising free speech.

The lawsuit, filed this week in federal court in Washington, alleges Trump is using his campaign to enforce nondisclosure agreements with former aides. The Trump campaign filed an arbitration claim against Sims last week, alleging he violated a non-disclosure agreement in writing the book.

Sims’ lawsuit, however, claims Trump’s campaign is trying to silence Sims by trying to apply an NDA to “information Mr. Sims learned solely during his federal service,” according to The New York Times.

Team of Vipers, which went on sale last month, claims Trump kept an “enemies list” and that counselor Kellyanne Conway is a closet leaker.

Sims told CNN anchor Anderson Cooper recently that he wasn’t concerned about being sued as a result of the memoir. Legal analysts have said NDAs like those Trump forces his aides to sign are unenforceable.

Sims told CNN he wasn’t sure whether he had signed an NDA, but probably made the same agreements as other former aides who wrote tell-all books, including press secretary Sean Spicer and 2016 campaign officials Corey Lewandowski and David Bossie.

Trump dismissed Sims’ book in a tweet, calling the author “a low level staffer that I hardly knew,” and claiming the book was “made up stories and fiction.”