Gameplay: Dying after nexus is destroyed

After destroying the nexus during a karthus game, I used the active on Zhonya's Hourglass. After the Hourglass's active stopped and I returned to normal, the nexus had been destroyed. Normally all champions become invulnerable, however in this situation, I was killed by the passive of a cinderhulk. I do not know if this is a bug. If it is please fix.