Amazon is restricting reviews of a new book about Prince Harry and Meghan, Duchess of Sussex, to verified customers, the Daily Mail reports.

Only users who have purchased the book from Amazon are allowed to review Omid Scobie and Carolyn Durand’s Finding Freedom: Harry and Meghan and the Making of a Modern Royal Family. The move came after a social media movement from fans of the couple to give the book positive ratings on the website.

Some critics have noted that the book is very favorable to its subjects. A reviewer for Kirkus wrote, “Though royal family members cannot authorize a biography, it’s hard to imagine a more supportive work than this.” And at the Washington Post, Autumn Brewington wrote, “For admirers of Harry and Meghan, Finding Freedom is 354 pages of sorbet.”

On Twitter, one user with the screen name “Henry VIII” wrote, “There is an organized effort to leave bad reviews for Finding Freedom on Amazon and to mark hateful reviews as ‘helpful’ so they appear first. Please leave a review, if nothing else mark 5-star reviews as ‘helpful.’”

Another user condemned the campaign, tweeting, “[S]orry, but just because you don’t agree with negative reviews—it doesn’t mean they should be deleted, removed or hidden.”

As of Tuesday morning, Finding Freedom had an average rating of four stars out of five on Amazon’s U.S. site, and 3.8 stars out of five on the retailer’s U.K. site.

Michael Schaub is a Texas-based journalist and regular contributor to NPR.