Unexpected error

[Christopher Eykamp <chris@...>]

I'm trying to run some pretty basic code, and am getting an unexpected and confusing error.  I'm hoping that more experienced eyes can tell me what's wrong.  This is standard code that was previously published, which I have not modified in any way.  I am running the latest version of Lua.

Here is the error:
C:\>lua listtest.lua
lua: listtest.lua:46: attempt to call method 'pushright' (a nil value)
stack traceback:
        listtest.lua:46: in function 'test'
        listtest.lua:72: in main chunk
        [C]: ?

C:\>

Here is the code:

-- $Id: lists.lua,v 1.6 2001/01/13 22:04:18 doug Exp $
-- http://www.bagley.org/~doug/shootout/
-- implemented by: Roberto Ierusalimschy

--------------------------------------------------------------
-- List module
-- defines a prototipe for lists
--------------------------------------------------------------

List = {first = 0, last = -1}

function List:new ()
  local n = {}
  for k,v in self do n[k] = v end
  return n
end

function List:length ()
  return self.last - self.first + 1
end

function List:pushleft (value)
  local first = self.first - 1
  self.first = first
  self[first] = value
end

function List:pushright (value)
  local last = self.last + 1
  self.last = last
  self[last] = value
end

function List:popleft ()
  local first = self.first
  if first > self.last then error"list is empty" end
  local value = self[first]
  self[first] = nil  -- to allow collection
  self.first = first+1
  return value
end

function List:popright ()
  local last = self.last
  if self.first > last then error"list is empty" end
  local value = self[last]
  self[last] = nil  -- to allow collection
  self.last = last-1
  return value
end

function List:reverse ()
  local i, j = self.first, self.last
  while i<j do
    self[i], self[j] = self[j], self[i]
    i = i+1
    j = j-1
  end
end

function List:equal (otherlist)
  if self:length() ~= otherlist:length() then return nil end
  local diff = otherlist.first - self.first
  for i1=self.first,self.last do
    if self[i1] ~= otherlist[i1+diff] then return nil end
  end
  return 1
end

-----------------------------------------------------------
-----------------------------------------------------------

-- Some tests

function test ()
  local SIZE = 10000
  -- create a list with elements 1..SIZE
  local l1 = List:new()
  for i=1,SIZE do
    l1:pushright(i)
  end
  -- creates a copy of l1
  local l2 = l1:new()
  -- remove each individual item from left side of l2 and
  -- append to right side of l3 (preserving order)
  local l3 = List:new()
  while l2:length() > 0 do
    l3:pushright(l2:popleft())  
  end
  -- remove each individual item from right side of l3 and
  -- append to right side of l2 (reversing list)
  while l3:length() > 0 do
    l2:pushright(l3:popright())
  end
  -- reverse l1 in place
  l1:reverse()
  -- compare Li1 and Li2 for equality
  -- and return length of the list
  if not l1:equal(l2) then return nil
  else return l1:length()
  end
end

N = tonumber((arg and arg[1])) or 1
for i=1, N do
  result = test()
end
print(result)