[Gameplay] bug - Eldar Dragon doesn't take damage

Just played a game where I was playing Olaf and my buddy vladamir. We were attempting to start a Eldar Dragon but it didn't take any damage. We hit it for a good 10 Seconds while I was I belive lvl 16 and he was lvl 17. No damage was done. When we pulled the Eldar Dragon out of the pit, it then took the normal amount of damage. Unfortunately, because we took so much damage trying to kill it, we had to retreat and the enemy team was able to kill it normally. This is the first time it's happend but it definately happened. If you want to send me 5,000,000 RP for finding this bug, that'd be great.