She was 13.

"Everyone here is really sad," an insider at Winfrey's Harpo production company told Celebrity Dog Watcher. "Sophie was truly part of the Harpo family as well as Oprah's."

Usmagazine.com said Winfrey talked about Sophie's illness in the December issue of her O magazine, revealing she and her boyfriend Stedman Graham were "preparing for the inevitable."

The couple has another cocker spaniel named Solomon. Their golden retriever, Gracie, died less than a year ago, Usmagazine.com said.