What is Supplied NxActorDesc is not valid.

[wallabie]

I started to get this error in the project recently and wondering what it means:

Supplied NxActorDesc is not valid. createActor returns NULL.
UnityEngine.GameObject:SetActive(Boolean)
NGUITools:SetActiveSelf(GameObject, Boolean) (at Assets/NGUI/Scripts/Internal/NGUITools.cs:1196)
NGUITools:Deactivate(Transform) (at Assets/NGUI/Scripts/Internal/NGUITools.cs:1085)
NGUITools:SetActive(GameObject, Boolean, Boolean) (at Assets/NGUI/Scripts/Internal/NGUITools.cs:1111)
NGUITools:SetActive(GameObject, Boolean) (at Assets/NGUI/Scripts/Internal/NGUITools.cs:1092)

[ArenMook]

I've never seen that,but a quick google search reveals http://forum.unity3d.com/threads/supplied-nxactordesc-is-not-valid.175607/