gameplay undestructible nexusus ARAM (playing cait)

i was playing aram at the late game i went backdooring to end the game and when i started autoing the nexus i couldn't do any damage to it even tho all my autos are landing after sometime the minions spwand and i killed some of them i started autoing the nexus but this time the nexus took damage i will link a replay that i made with baronreplay please attend to this proplem as soon as you can sorry for the bad English

http://www.mediafire.com/download/06mw6502muvgdz9/bug_report_.lpr