- They left a sweet message wishing him a happy birthday: "Happy Birthday Louis! Sending lots of love from both of us!"
- Some commenters think the couple broke royal protocol by not addressing him as "Prince Louis."
- This appears to be one of the first times the Sussex Royal account has interacted with Kate Middleton and Prince William's Instagram account.
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Meghan Markle and Prince Harry caused a stir when they set up their own Instagram account in April. The Duke and Duchess of Sussex's official account has been used to share photos of their royal duties, and more recently to send a sweet birthday message to their nephew, Prince Louis.
When the palace posted a set of photos taken by Kate Middleton herself, the Sussex Royal account left Louis a special birthday comment.
"Happy Birthday Louis!" they wrote in the comment, "Sending lots of love from both of us! xo"
While the comment seemed to delight many royal fans based on the 7,204 "likes" it has received at the time of this post, some weren't too pleased that the couple or whoever wrote the message for them left out the word "Prince" when addressing their nephew.
"Shouldn't you be using his title because you're addressing him in public?" one commenter wrote while another person added, "It's not just Louis ... It's Prince Louis of Cambridge. Please be formal."
Others defended Markle and Harry's comment, claiming that "it's Instagram, they don't have to be formal," and asking, "why would it be Prince Louis to his uncle and aunt?"
This appears to be one of the first times fans have seen them interact with Middleton and Prince William's official account, as the couple has largely been using their Instagram account to keep royal watchers updated on their philanthropic duties and to share previously unreleased photos.
Kensington Palace and royal experts did not immediately respond to INSIDER's requests for comment.