Harry and Meghan responded to the post from their respective page, Sussex Royal, wishing their nephew the best.
"Happy birthday Louis! Sending lots of love from both of us xo."
But fans were less than impressed with how the Harry and Meghan referred to their tiny royal relative and hit back at the couple for failing to use the Prince's official title.
"@sussexroyal if you don't believe in titles that's fair enough, but then don't expect people to use yours to address you," one user wrote, per Express.co.uk.
Another said: "It's PRINCE Louis to you. He outranks you both!"
Prince Louis is fifth in line to the throne, after his grandfather Prince Charles, dad William, brother Prince George and sister Princess Charlotte.
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