Recently, Selena Gomez celebrated her 21st birthday in L.A., inviting all of her closest friends to the party. So why wasn’t her best friend Taylor Swift there?

According to The Hollywood Life, 23 year-old Swift avoided the party because Selena’s ex, Justin Bieber, was going to be there. A source told Hollywood Life that,

“[Taylor’s] not going to want to be there if Justin is there, it’s so awkward between them now.”

It’s been clear to the world that Swift has a certain distaste for the “Boyfriend” star, especially after this year’s Billboard Music Awards when Swift stuck her tongue out at Bieber while him and Gomez embraced back staged and shared a cheek-to-cheek kiss.

Granted, Bieber wasn’t the only reason why Swift didn’t show up to her best friend’s party. Swift has been performing on the East Coast and would be on an extremely tight schedule if she were to fly out to L.A. after one of her performances. On top of that, Swift has been described as a “homebody,” as someone who isn’t a huge partier and doesn’t drink often.

Although she missed the big “hoopla,” Swift did say that she was planning on making it up to her best gal pal.