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Michael Douglas has been known to say some out-of-pocket things before — such as the (debunked) NSFW reason he claimed caused his throat cancer — and his most recent Instagram post is no exception.

In a Wednesday, September 25 upload, the 80-year-old Wall Street star referred to wife Catherine Zeta-Jones as his "sister" while honoring the Chicago actress on her 55th birthday.

"To my birthday sister, I love you with all my heart! May your new year be the best! @catherinezetajones ❤️," Douglas wrote, alongside a black-and-white photo of the Oscar winner.

While Zeta-Jones loved the tribute — "🎉 Thank you sweetheart!!!🎉🎉🎉🎉🎉 🎉🎉🎉🎉🎉," she replied, and reposted her husband's post to her Instagram Story — fans in the comments section were stunned.

"Why would you refer to your wife as a sister???" one person asked. "Sister??🤨🧐," another questioned. "happy birthday to your queen, not your sister 😂," a third person commented.

Michael Douglas and Catherine Zeta-Jones walk the red carpet during the 71st Annual Primetime Emmy Awards on September 22, 2019 in Los Angeles, California. Douglas left fans extremely confused when he referred to Zeta-Jones as... Michael Douglas and Catherine Zeta-Jones walk the red carpet during the 71st Annual Primetime Emmy Awards on September 22, 2019 in Los Angeles, California. Douglas left fans extremely confused when he referred to Zeta-Jones as his "sister" — and not his wife — in a birthday tribute in September 2024. Rich Polk/Getty Images

As it turns out, Douglas and Zeta Jones share the same birthday — albeit 35 years apart — hence the "birthday sister" comment. (Perhaps "birthday twin" would have been more accurate.)

Zeta-Jones, for her part, returned the favor and paid tribute to her husband on his 80th birthday.

"Happy Birthday to Michael and to me!!" she wrote alongside a throwback photo of the pair, who have been married since 2000. "This was us on our birthday 25 years ago! I love you with all my heart ♥️, our friend @jeanpigozzi snapped us in New York!!"

Notably, she also referred to Douglas as her spouse when sharing another celebratory post on her Instagram Story.

"Happy bIrthday to my husband ♥️," she wrote over a black-and-white photo of the Fatal Attraction actor.

Zeta-Jones and Douglas share two adult children together: son Dylan, 24, and daughter Carys, 21. Douglas is also father to his eldest son, 45-year-old Cameron, whom he shares with ex-wife Diandra Luker. The exes were married for 23 years before finalizing their divorce in 2000.

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