GUI NullReferenceException

The minute I create a GUI skin I get:

"NullReferenceException: Object reference not set to an instance of an object UnityEngine.GUISkin.BuildStyleCache () UnityEngine.GUISkin.OnValidate () UnityEngine.ScriptableObject:.ctor() UnityEngine.ScriptableObject:.ctor() UnityEngine.GUISkin:.ctor() GUISkinEditor:CreateNewGUISkin()

I added var customSkin : GUISkin; to my script as per my understanding of the documents and selected my GUI skin in the Inspector but the error remains. Plus my custom settings don't seem to be working so I guess it's not the right script. But why does the NullReferenceError generate the second I create a GUI skin?