If the item you are making doesn't replace a slot that Valve designates, it won't work. I suggest making an item for witchdoctor in an item slot designated by Valve so you won't run into this error. Of course Valve might have made a mistake as they somtimes do, but I haven't checked out witchdoctor's assets yet, so I can't be sure.

Well, they don't tell which bone to assign the belt actually, and there is no real "belt" bone, so i went for the closest, tried several others, but even if i am still at the wrong bone, WHY is the basic test model of witch doctor differ, from the basic model which you can see in the loadout without any equipment? I mean, makes no sense. And i am not talking about what you can download model, i am talking fully ingame models. When you upload your item to the workshop and you can see it on the character, THAT character has no loincloth and bag instead:) I was confused and checked the loadout of my witch doctor unequipping everything, loincloth one without bag (as i remembered). So what the heck then? Also, i can make then a weapon for him, but that's not what i wanted to. Can't make only weapons, just because we only know for sure which one is the weapon slot:P

Thanks for the answer though, they didn't write anything back even on the developer's forum yet..