Skinning and -1 index into joints array


I’m doing skinning (applying bind shape to mesh) when converting to some binary format.
I have problems understanding this part of specification about <vertex_weights> :

"<v> - Contains a list of indices that describe which bones and
attributes are associated with each vertex. An index of -1
into the array of joints refers to the bind shape
. Weights
should be normalized before use. This element has no
attributes. "

Here is an example of a more complete <vertex_weights> element. Note that the <vcount> element
says that the first vertex has 3 bones, the second has 2, etc. Also, the <v> element says that the first
vertex is weighted with weights[0] towards the bind shape, weights[1] towards bone 0, and
weights[2] towards bone 1:

So, what node should be used if I found -1 as and index in a list of influences (<v>)? My target format requires that each influence addresses some bone. Should I constuct some dummy node with transformations from bind_shape_matrix or do something other?