I bugged reported this recently - but if its just the default nwn spell version of this and working as intended, It could be cool to change the spell so that it is basically animate dead but no corpse is required and maybe you could select a corpse type from a menu.. ogre/orc/warrior bones.. etc. This could even theoretically be built into some rare necromancer loot items and possibly into the necro perks at level 8 and would provide a great perk to your 8/10 str necromancers who are permanently encumbered.