ERROR with util.AddNetworkString

[Melonos]

The error is:

[ERROR] addons/firearmdealer/lua/autorun/sv_mainnpc.lua:20: attempt to call field 'AddNetworkString' (a nil value)
  1. unknown - addons/firearmdealer/lua/autorun/sv_mainnpc.lua:20

MyCode:

function spawnNPC()
		--> Dealer
		local npc = ents.Create("base_ai")
		npc:SetPos( NPCPos + Vector(0, 0, 10))
		npc:SetAngles( NPCAng )
		npc:SetModel( NPCModel )
		npc:SetHullType(HULL_HUMAN)
		npc:SetHullSizeNormal()
		npc:SetNPCState(NPC_STATE_SCRIPT)
		npc:SetSolid(SOLID_BBOX)
		npc:CapabilitiesAdd(bit.bor(CAP_ANIMATEDFACE, CAP_TURN_HEAD))
		npc:SetUseType(SIMPLE_USE)
		npc:Spawn()
		npc:DropToFloor()
			--> Input
	function npc:AcceptInput(name, activator, caller)
		if name == "Use" and IsValid(caller) then
				net.Start("MelonosPanel")
					net.WriteInt(1)
				net.Send(caller)
		end
	end

end

hook.Add("InitPostEntity", "Melonos.spawnNPC", spawnNPC)

[SteppuFIN]

The file sv_mainnpc is running shared. It should be in lua/autorun/server/sv_mainnpc.lua

[Beast]

You have the file as a shared file, and util.AddNetworkString is a serverside function. Just make the file serverside only, and you should be set.