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Nicole Richie, Paris Hilton

Gregg DeGuire/WireImage.com

When Nicole Richie got hitched to Joel Madden this weekend in front of friends—such as Gwen Stefani, Ashlee Simpson-Wentz and Khloé Kardashian Odom—at dad Lionel's Beverly Hills estate, many were wondering why Paris Hilton wasn't there.

The former BFFs and Simple Life costars did have a falling out years ago, but have since patched things up.

And although Page Six says Paris didn't make the guest list for the intimate affair, a Hilton confidant insists that's not the case...

"She was invited, but she had another commitment," a source close to the heiress tells me.

So what was the reason she couldn't be there to see Nicole tie the knot?

Paris was shooting her Oxygen reality series in Vegas this weekend, where one of her castmates was celebrating her birthday (costar Brooke Mueller didn't attend).

"This was planned months ago," the source says. "Paris sent Nicole and Joel a wedding gift to congratulate them and wishes them both the best."

Despite not being in attendance, Paris didn't seem too bummed out about missing Nicole's big day.

"Good morning everyone! Off to Vegas for my BFF @CamRaFace's Birthday! Huge! Loves it! :)," she posted on her Twitter on Saturday.

Another reason she may have been relieved to sit this one out? Benji Madden, her former boyfriend and brother of the groom, would obviously in attendance. And it's never fun being around exes, right?