Katy Perry sad about missing Rihanna's birthday party

Rihanna has no problem with close friend Katy Perry skipping her birthday bash - because she's still not forgiven herself for missing her pal's wedding to Russell Brand.

The Umbrella hitmaker turned 23 on Sunday (20Feb11) and marked the occasion with a massive party.

In a post on her Twitter.com page, she writes, "Go grawl, its ya birthday, ITS PARTY TIME! All my best friends are here!!!! And my BRO!! Wish u cud see how SICK this is!!! Stay tuned."

Perry, who is currently on tour in Europe, replied: "Happy birthday boo. Wish I could b there to blackout! Luv U!"

But Rihanna told Perry she shouldn't be sad about skipping the celebrations, insisting missing Perry's October (10) wedding to the British funnyman was a bigger snub.

The Bajan beauty adds, "Are u kidding? I missed ur WEDDING!! This is small stuff boo! When work calls."