Error: Bone weights do not match bones.

I am currently combining a lot of skinned meshes and then copying the blend shapes.

After the whole process is done, if I try to use any of the blend shapes (their values are not zero) I keep getting spammed this error with absolutelly no details. It is probably coming from the C++ part of the engine or something.

All bones are working perfectly (unless I dare use any blend shape, then it just stops updating and stays where it was last)

 

This happens if any of your bones weights points to the bone -1.
The mesh will animate properly (probably because coincidently, all -1 bones had weight of 0, I can't be sure). But the blend shapes simply can't handle this.
So be weary of using Array.IndexOf when fusing many skinned meshes together, as it will silently set a bone to -1 if it's not found due to a bug somewhere else