How to return npc's uid?

[Cosmotonio]

Hi, i need to return npc's uid, look this.

pos ={x=selfGetPosition().x,y=selfGetPosition().y,z=selfGetPosition().z, stackpos = STACKPOS_TOP_CREATURE}
npcuid = getThingfromPos(pos).uid

error:

[16/12/2009 09:16:12] Lua Script Error: [Npc interface] 
[16/12/2009 09:16:12] data/npc/scripts/treinador.lua:onCreatureSay

[16/12/2009 09:16:12] data/npc/scripts/treinador.lua:46: attempt to index a number value
[16/12/2009 09:16:12] stack traceback:
[16/12/2009 09:16:12] 	data/npc/scripts/treinador.lua:46: in function 'callback'
[16/12/2009 09:16:12] 	data/npc/lib/npcsystem/npchandler.lua:374: in function 'onCreatureSay'
[16/12/2009 09:16:12] 	data/npc/scripts/treinador.lua:8: in function <data/npc/scripts/treinador.lua:8>

 

[chojrak]

getNpcCid()