IU surprised Seulong on his birthday.
On May 10, Seulong thanked IU via Instagram with a photo and caption that said, "A cake for a birthday that I was just going to let it pass quietly. You, who made a surprise appearance with a cake, are really the best :) "
SEE ALSO: KARD x AKP Collab Merch!
The two singers have collaborated for the hit track ''Nagging'' in 2010 and have maintained a tight bond ever since.