An Atlanta-based private equity investment firm is suing longtime legal counsel Alston & Bird over claims that Alston lawyers provided bad legal advice based on a multimillion-dollar math error—then allegedly attempted to cover it up.

MSouth Equity Partners filed the legal-malpractice suit against Alston to claw back more than $7.6 million in federal and state taxes and interest the investment firm said it was forced to pay to the IRS following a 2011 investment deal involving another Alston client in which MSouth held a majority interest and on which Alston lawyers provided legal advice, the malpractice complaint contends.