Zombies are attacking the empty space and ignore the player.
The first time I heard that someone is attacking zombies appeared shaken by air, the second time that I saw someone running zombies ka zombies eventually ran to a certain place, and beat into the void.
http://u.to/1xF2BQ translate google ru
http://youtu.be/aEJH0v-Opx4?t=5m22s {F29094} {F29095} {F29096} {F29097} {F29098}