were do i report a annie stun BUG

some times when i have the four stacks on my stun and i use my Q on a champion, it doesn't stun them but the stacks will dissapear. I already checked the last time it happend if the enemy (that time it was cho'gath) didn't have banshee, but this happend at lv 3 (him and me) so they didn't have it. can some on tell me were i can report this?