A Washington D.C.-based consulting firm that worked with disgraced former national security adviser Michael Flynn’s foreign influence outfit is being sued by its own lawyers for more than half a million dollars.

The firm, Lewis Baach, filed a lawsuit last month alleging that the consultancy, Sphere Consulting, is delinquent on $572,000 in legal bills for work dating to 2015. Sphere Consulting flatly denies the charges.

The lawsuit is just the latest bit of drama for Sphere, which last year found itself fielding questions about its work with Flynn as part of the public relations work it did on behalf of a Turkish government-linked client. Resulting allegations of federal lobbying violations eventually led to Flynn’s resignation from the White House. He has since pleaded guilty to lying to federal prosecutors investigating Russian influence in the 2016 election. Sphere was not accused of any wrongdoing stemming from its work on Flynn’s behalf, but it cooperated with Special Counsel Robert Mueller’s investigation into Flynn prior to the guilty plea.

It’s not entirely clear what is at the heart of the consulting firm’s dispute with Lewis Baach, which was first reported by Politico. However, a top official for Sphere hinted that it was a matter of overbilling.

“Sphere Consulting has been a major source of business referrals to Lewis Baach and is absolutely stunned with the value they placed on their services,” said Jim Courtovich, Sphere’s founder and managing partner, in an email.

Since filing its lawsuit, Lewis Baach has offered to settle for $275,000. Courtovich countered with a $200,000 offer, which the law firm rejected. Sphere has yet to file an official answer to the firm’s complaint.

Lewis Baach did not respond to a request for comment on the lawsuit.