Michael Douglas got a strong reaction from fans after calling his wife, Catherine Zeta-Jones, his “birthday sister.”

The couple, who interestingly share a birthday on September 25 despite being born 25 years apart, took to social media on Thursday to celebrate each other’s birthdays. However, Douglas’ heartfelt tribute received a range of responses.

“To my birthday sister—what a joy celebrating another year with you!,” he captioned a selfie of the two playing golf.

“Congratulations on your outstanding performance at the Celebrity Ryder Cup,” the 81-year-old actor went on to say. “Happy Birthday to the love of my life @catherinezetajones ❤️ .”

“That’s his wife not his sister, what do he mean birthday sister?” one person wrote in the comments section.

Another fan asked, “Did you just call your wife sister?? Or am I tripping??”

Many fans appeared to be confused.

“That’s his wife not his sister, what do he mean birthday sister?,” another comment reads.

In her message to her husband, the 56-year-old actress expressed, “Happy Birthday Michael! Celebrating this unique day with you is truly a once-in-a-lifetime experience!😘.”

The couple’s 22-year-old daughter, Carys, shared a sweet message for her parents as well.

“September 25th 🤍Happy Birthday Mom and Dadda!!” she wrote, sharing a picture of the “Basic Instinct” star holding both her and her mother’s hands. “Your love of life, love, learning, art… there are no words to describe my gratitude and admiration.”

Douglas and Zeta-Jones have been married since November 2000, and aside from Carys, also share their 25-year-old son, Dylan.

Douglas also has a 46-year-old son, Cameron, whom he shares with his ex-wife, Diandra Luker.

Last month, Zeta-Jones talked about sharing a birthday with Douglas and being alike in an interview with the Sunday Times.

“We respect our space, we’re independent spirits,” she shared. “We are very similar; we were born on the same day, 25 years apart.”

“We’re not afraid to be verbal, to express ourselves,” she continued. “I wear everything on my sleeve and so does he, which is good.”