the skin may have a bone which has been deleted. check the bone names against nodes in the scene. Bones in a skin modifier are handled odd, and even if a bone is removed, the index still remains. If you need to, repaint the skin from scratch.
Alternately, sometimes you can add another bone to fill the missing index. You can then export the model and check the skin-bone section for weird numbers, zeros, or bones without proper names.
