Reviving an old script error: Attempting to index a nil value

local cells = {}

local function addCells(...)
	for i = 1, #arg do  -- "arg" is a numerically indexed table containing everything passed to the function (wrapped peripherals, in this case).
		cells[#cells + 1] = arg[i]
	end
end

-- http://www.computercraft.info/wiki/Peripheral.find
addCells(peripheral.find("tile_thermalexpansion_cell"))  -- Wrap all available peripherals of type "tile_thermalexpansion_cell", pass the lot to addCells().
addCells(peripheral.find("powered_tile"))
addCells(peripheral.find("capacitor_bank"))

-- "cells" now contains all available peripherals of the above types, wrapped.

local monitors = {peripheral.find("monitor",
	function(name, object)  -- If this function returns false when passed a found monitor, it won't be included in peripheral.find's results.
		local xSize, ySize = object.getSize()
		return xSize > 38 and ySize > 18 and object.isColour()
	end)}
	
-- "monitors" now contains all available colour-capable monitor peripherals that're at least 39x19.
-- So we can do stuff like monitors[1].write("whatever"), monitors[2].setTextColour(colours.whatever), up to however many monitors we have...

-- Leaving just the turbines and reactors:
local turbines = {peripheral.find("BigReactors%-Turbine")}
local reactors = {peripheral.find("BigReactors%-Reactor")}

--Check for storage cells and monitors before continuing
if #cells == 0 then    -- #cells returns the number of entries in the cells table.
    print("No RF storage found. Exiting script!")
    return
end

if #monitors == 0 then
    print("No Monitor found. Exiting script!")
    return
end

--Print connected peripherals
print("Peripherals connected:")
if #monitors == 1 then print("1 Monitor") else print(#monitors.." Monitor") end
if #cells == 1 then print("1 Cell") else print(#cells.." Cells") end
if #turbines == 1 then print ("1 Turbine") else print (#turbines.." Turbines") end
if #reactors == 1 then print ("1 Reactor") else print (#reactors.." Reactors") end