The singer/actress recently shared a series of photos from London, where she is currently starring in the West End version of hit Broadway musical "Waitress", and one follower decided to use the comments section to criticise Katharine for her relationship with the 69-year-old musician.
"David's cheating on you," they wrote. "you're such a fool".
The claim caught David's eye and he decided to take the fellow social media user to task.
"wow that's a big accusation," he wrote, as he denied the gossip. "how bout giving us a name-I'm sure u (sic) wouldn't mind outing 'her' like you falsely did me!! I'm calling you out!!"
And the drama didn't appear to faze Katharine, 35, who dismissed the troll's remark by posting, "Bye honey. Go bark up someone else's tree."
The couple became engaged last summer (18) after a year of dating.
The union will mark David's fifth time down the aisle, after divorcing his fourth wife, former model Yolanda Hadid, the mother of runway beauties Gigi Hadid and Bella Hadid, in 2017 after six years of marriage.
The bride-to-be is also no stranger to weddings - she was married to Nick Cokas from 2008 to 2016.