Julie Andrews playing big fish in Aquaman

Julie Andrews has landed a secret role in new DC Comics blockbuster Aquaman.

Sources tell EW.com the actress is making an appearance in the superhero film, which will be released the same week as Mary Poppins Returns

— the sequel to Julie’s beloved 1964 Disney movie.

Andrews will voice Karathen, a mythical undersea creature, in the film, which stars Jason

Momoa as Aquaman, aka Arthur Curry.

“We wanted the Karathen to have the voice of a classic British actress, albeit somewhat digitally altered, and when we found out Julie was interested and available and excited to do it, casting her was a no-brainer,” producer Peter Safran tells EW.com. “Who knew that Aquaman would have the real Mary Poppins this Christmas?”

The blockbuster arrives in cinemas on Dec. 21.