ARAM Nexus Taking No Damage [GAMEPLAY]

In ARAM our Caitlyn sneaked past the enemy team and attacked the open nexus (inhibitor was down). The attack animation went off, but the nexus took no damage whatsoever. She had about six or seven auto attacks until she was killed, but the nexus was apparently not hit.

I don't know if this can be reproduced, but the steps were something like this:

1. Play Caitlyn
2. Have an open enemy nexus with the inhibitor down
3. Attack it with auto attacks while the enemy team is in your back and minions spawn and attack you