UE3 - UT3 Terrain Error: "Invalid MaterialIndex"

I have a map using a few different terrains. I've had this issue for a while, but now am trying to resolve it. Its not having an effect on the map itself, its just an annoying and unprofessional error when the map is built.

The error is:
"Terrain> Invalid MaterialIndex 1 - mapname.TheWorldersistentLevel.Terrain_10"

I have looked at some other questions about this problem, some of whom say to uncheck the 'highlighted' option for all the layers, but this has not worked.
I have tried deleting all the layers (I am willing to repaint the terrain to get round the problem), but no luck there either.

Any ideas?

Problem solved.

For any of those who have this problem (and may get to this page via google), here is how I eventually fixed it:

- Delete all layers on each terrain.
- Delete all terrain-layer-setups in your package.
- Delete all terrain-materials in your package.
- Re-paint your terrain with new layers/materials

At least you don't have to make the terrain heightmap from scratch =)