Just as Rihanna's career stood in the way of her being Maid of Honour at Katy Perry's wedding to Russell Brand, Katy's career has prevented her from attending Rihanna's birthday celebrations.
Katy has embarked on her California Dreams world tour and was in Europe, unable to help her BFF celebrate her 23rd birthday on Sunday.
RiRi took to Twitter to express her excitement, writing: "Go grawl, its ya birthday, ITS PARTY TIME! All my best friends are here!!!! And my BRO!! Wish u cud see how SICK this s**t is!!! (sic)" she wrote.
Katy tweeted: "Happy birthday boo. Wish I could b there to blackout! Luv U!"
Not wanting to let her friend feel bad, Rihanna replied: “Bitch are u kidding? I missed ur WEDDING!! This is small stuff boo! #whenworkcalls."
We love these celeb BFF's!