Many years ago, Stevie Nicks made a choice to never be married and never have children, saying that doing so would get in the way of her true calling. But the Fleetwood Mac singer did get married -- for three months, to her best friend's husband -- an experience Nicks opens up about in this clip from "Oprah's Master Class."

The story of Nicks' marriage begins in 1981, when her best friend, Robin, was diagnosed with leukemia. While in remission, Robin discovered that she and her husband, Kim, were expecting a child. Robin's cancer returned, however, and she discontinued treatment. "She knew that she was going to die," Nicks says in the clip, "and she wanted to leave that baby behind. She wanted something to be left from her."

Soon after Robin's child, Matthew, was born, Robin passed away. Nicks was devastated by her best friend’s death, and the grief sent the singer on a mission. "I wanted that baby," she says in the clip. "I convinced Kim, and three months after Robin died, we got married."

Nicks says she thought it was what Robin would have wanted, but it soon became clear to her that not everyone agreed with the decision. "It just blew up in our faces," says Nicks -- who now understands why others disapproved. "It was so the wrong thing. To take Matthew and [Robin’s] husband was so beyond insane."

In the clip, Nicks also recalls feeling Robin's presence in Matthew's nursery, and shares how she told Kim that she wanted a divorce. Even now, the ill-advised marriage is something Nicks has trouble explaining. "Even… sitting here right now, [I'm wondering], 'What were you thinking?'" says Nicks. "I wasn't thinking. And that's the story of my marriage."

"Oprah’s Master Class" airs on Sundays at 10 p.m. ET on OWN.

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Earlier on HuffPost:

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  • LONDON, ENGLAND - JUNE 26: Stevie Nicks performs live on stage during the third day of the 'Hard Rock Calling' music festival at Hyde Park on June 26, 2011 in central London, England. (Photo by Jim Dyson/Getty Images)

  • NEW YORK, NY - MARCH 26: Stevie Nicks performs at Madison Square Garden on March 26, 2011 in New York City. (Photo by Cory Schwartz/Getty Images)

  • NEW YORK - MARCH 19: Stevie Nicks of Fleetwood Mac performs at Madison Square Garden March 19, 2009 in New York City. (Photo by Rob Loud/Getty Images)

  • MELBOURNE, AUSTRALIA - NOVEMBER 01: Former Fleetwood Mac Singer Stevie Nicks attends a press conference at The Melbourne Cup at Flemington Racecourse November 1, 2005 in Melbourne, Australia. (Photo by Patrick Riviere/Getty Images)

  • NEW YORK - SEPTEMBER 28: Singer Stevie Nicks performs on theToday Show for 'Make a Difference Today' at Humanity Plaza on September 28, 2005 in New York City. (Photo by Peter Kramer/Getty Images)

  • Stevie Nicks on 'The Tonight Show with Jay Leno' at the NBC Studios in Burbank, Ca. 8/20/01. Photo by Kevin Winter/Getty Images.

  • Stevie Nicks performing at a private show to introduce her upcoming May 2001 release 'Trouble in Shangri-la', her first solo collection since 1994, at SIR Studios in Los Angeles, California, March 31, 2001. Photo by Frank Micelotta/ImageDirect.