A Houston law firm specializing in estate planning is facing a legal malpractice lawsuit from clients who allege their counsel’s error on a tax statement triggered an Internal Revenue Service audit that cost them more than $628,000 in penalties and fees.

The plaintiffs, Marc and Elizabeth Walraven, Tarrant County residents, filed their lawsuit in Harris County district court Aug. 5 against Richard C. Shanks and his firm, Shanks & Hauser.