Inhib invulnerability bug

Last night I was playing as jungle Skarner and split pushing the bot lane to take the inhib while there was a small skirmish mid. I had devourer, triforce, randuins, and I think at that time probably sunfire as well. Anyway, when I got to the inhib with my minion wave I couldn't actually hurt the inhib. I was completing the attack animations but the structure did not take damage. The minions around me could hurt it but I sat there attacking about 20 times hoping to see it was just lag, it was not. Anyone else seen this bug?