RebuildVertexColors issue with TMK

Yes, this is a real question. I had TMK working fine and dandy, but I've made some script changes to my project since last time I ran it - GUI stuff, and commenting out the calls to the TMK initialisation stuff so I can take screenies with/without the shiny stuff to compare. Now my level has gone super-bright, and when I look at an interior, I get a fatal assert in rebuiltVertexColors - the pNormalMap (something like that) has an invalid bitmap, it looks like. What did I do?
EDIT: Actually, the problem seems to be caused irregularly. Sometimes it happens when the character hits the ground, other times soon after, but I never get a glimpse of an interior.

EDIT: I'm really good with the stupid questions. Having a sky fixed the issue...