London law firm Russells is probably wishing that it had used a silencing charm on its attorneys, after it was revealed that one of the firm’s partners exposed J.K. Rowling as mystery writer Robert Galbraith.

The firm admitted on Thursday that Chris Gossage, a partner who has worked with entertainment industry clients, told his wife’s best friend that the “Harry Potter” creator was the true author behind “The Cuckoo’s Calling.” The detective novel, which was published to critical acclaim earlier this year, had been billed as a debut novel from Galbraith.

Upon learning of Rowling’s pseudonym, that friend, Judith Callegari, used Twitter to tip a Sunday Times columnist about the news.

In a statement, Russells’ apologized “unreservedly” for the leak. “Whilst accepting his own culpability, [Gossage made] the disclosure in confidence to someone he trusted implicitly,” the firm said, according to the New York Times. “On becoming aware of the circumstances, we immediately notified J. K. Rowling’s agent. We can confirm that this leak was not part of any marketing plan and that neither J. K. Rowling, her agent nor publishers were in any way involved.”

In her own statement, Rowling said she was “very angry” that Russells had broken her trust.

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