Need help!

Here is what I do:
-import mount_horse_light_XXX.CAS
-select Mesh -> Skin -> Envelope
-open Weight Table
-give all Vertices from 'bone_H_Saddle' to 'bone_H_Lower back'
-export CAS

Every time I try to load this saved CAS it comes up with an error.
The same when I give only one Vertex to the other bone.

Any suggestion?

Thanks in advance!