arithmetic __sub on nil and number error

--Variable chunk excavating depth program, doesn't include auto disposal of blocks however.
local function DigOneRow()
for i = 1,15 do
  while (turtle.detect()) do
   turtle.dig()
   sleep(0.5)
  end
  turtle.forward()
end
end
local function DigTwoRows()
DigOneRow()
turtle.turnRight()
while (turtle.detect()) do
  turtle.dig()
  sleep(0.5)
end
turtle.forward()
turtle.turnRight()
DigOneRow()
turtle.turnLeft()
while (turtle.detect()) do
  turtle.dig()
  sleep(0.5)
end
turtle.forward()
turtle.turnLeft()
end
local function LastRowsInLayer()
DigOneRow()
turtle.turnRight()
while (turtle.detect()) do
  turtle.dig()
  sleep(0.5)
end
turtle.forward()
turtle.turnRight()
DigOneRow()
end
local function MoveToStartXZ()
turtle.turnRight()
DigOneRow()
turtle.turnRight()
end
local function DigOneLayer()
for i=1,7 do
  DigTwoRows()
end
LastRowsInLayer()
MoveToStartXZ()
turtle.digDown()
turtle.Down()
end
local function UserDepthInput()
print("Hello. I am Shaft001. I can dig a 16x16 hole to the depth you desire. Please note that I have no checkFuel function so you must chock me full of fuel before starting! If digging a deep shaft, note that I do not have an automatic item storage function so you must periodically empty my inventory.")
print("Please enter a value, preferably under 20, but must be less than current Y value-4.")
print("Dig Depth:")
depth= read("")
print("Your selected depth is: "..depth)
end
local function InputtedDepthLayers()
local depth=depth
local depthup=depth-1
for i=1,depth do
  DigOneLayer()
end
turtle.Up()
turtle.select(1)
turtle.placeDown()
for i=1,depthup do
  turtle.Up()
end
end
InputtedDepthLayers()