Walking Nexus didn't spawn in Nexus Blitz

I'm not sure as to the exact circumstances for this happening, as i've played about 100 games of nexus blitz and this is the first time it happened, but my team's walking nexus spawned with the healthbar showing at the top and the enemy team's never walked into lane, and the health bar at the top showed up as 0. The tower/inhib registered that there was a walking nexus, making it so we can't damage enemy structures, and thus making it impossible to win.