LUA ERROR: sys/lua/cs2dtibia/npcs.lua:594: attempt to index field '?' (a nil value)

               text = text .. ITEMS[itemid].name .. "|" .. l[2] .. ","

for i, j in ipairs(NPCs) do
     j.image = image("gfx/weiwen/" .. (j.image or "npc1") .. ".png", 0, 0, 0)
     imagepos(j.image, j.pos[1], j.pos[2], j.rot)
     if j.trade then
          local text = j[1] .. ","
          for k, l in ipairs(j.trade) do
               local itemid
               if l[1] < 0 then
                    itemid = -l[1]
                    text = text .. "sell "
               else
                    itemid = l[1]
                    text = text .. "buy "
               end
               text = text .. ITEMS[itemid].name .. "|" .. l[2] .. ","
          end
          j.menu = text
     end
end