Attempt To Index ? (a Nil Value)

local args = {...}

if #args ~= 4 then
  print("Usage: build <file> <x> <y> <z>")
  exit()
end

local file = args[1]
local x = args[2]
local y = args[3]
local z = args[4]

sfile = fs.open(file, "r")
Instructions = split(sfile.readAll(), ",")
if Instructions == nil then
  print("Instructions file is corrupt or empty")
  exit()
end

--shell.run("goto", x, y, z)

for i = 1, #Instructions do
  local slot = tonumber(split(Instructions[i], "#")[1])
  local coords = split(split(Instructions[i], "#")[2], ";")[1]
  local bx = currentX + coords[1]
  local by = currentY + coords[2]
  local bz = currentZ + coords[3] + 1

  shell.run("goto", bx, by, bz)
  turtle.select(slot)
  turtle.placeDown()
end

--shell.run("goto", x, y, z)

function split(pString, pPattern)
  if sep == nil then
    sep = "%s"
  end
  local t = {}
  local i = 1
  for str in string.gmatch(pString, "([^"..pPattern.."]+)") do
    t[i] = str
    i = i + 1
  end
  return t
end