Lua moving table up causing nil value error - indexing

Question

I have a table of tables which represents a 65*65 square grid where each element is integer values. It represents the area around an actor and the actor can move in the four cardinal directions and as they do I need the table to move with them so they remain in the center square 33,33.
Rather than moving the elements of the table I instead have two pointers tableIndex.left and tableIndex.down which store the indexes of the far left column of the bottom row respectively, they both start at 65. I have three helper functions to perform operations on the table, all take the arguments x1,x2,y1,y2 where x1 and y1 are the co-ordinates to start the function at and x2 and y2 are the co ordinates to end the function.
These functions are tableInit which creates tables in all the values between x1 and x2. zero table which fills all of the entries in the specified range with 0 and clear table which sets all the entries in the specified range to nil.
function movetable(direction)
--function to move our probability histogram
-- 0 is up, 1 is right, 2 is down, 3 is left
--as a note robotSpeed = 1
if direction == 0 then
tableIndex.down += robotSpeed
zerotable(tableIndex.left,tableIndex.left+64,tableIndex.down+64,tableIndex.down+64)
cleartable(tableIndex.left,tableIndex.left+64,tableIndex.down-1,tableIndex.down-1)
elseif direction == 1 then
tableIndex.left += robotSpeed
tableInit(tableIndex.left+64,tableIndex.left+64)
zerotable(tableIndex.left+64,tableIndex.left+64,tableIndex.down,tableIndex.down+64)
cleartable(tableIndex.left-1,tableIndex.left-1,tableIndex.down,tableIndex.down+64)
elseif direction == 2 then
tableIndex.down -= robotSpeed
zerotable(tableIndex.left,tableIndex.left+64,tableIndex.down,tableIndex.down)
cleartable(tableIndex.left,tableIndex.left+64,tableIndex.down+65,tableIndex.down+65)
else
tableIndex.left -= robotSpeed
tableInit(tableIndex.left,tableIndex.left)
zerotable(tableIndex.left,tableIndex.left,tableIndex.down,tableIndex.down+64)
cleartable(tableIndex.left+65,tableIndex.left+65,tableIndex.down,tableIndex.down+64)
end
end
The table is called later in the program,in the following section of code, which is where the error is thrown "Attempt to perform arithmetic on field '?' a nil value. This occurs when i and j are 0. This only occurs after the table has last moved up so dir==0 and the error ceases to exist if I comment out the cleartable call in my movetable function for the case when dir==0.
for i=0,64 do
for j=0,64 do
--Robot is at 33 33 so we know there isnt an obstacle here
if i ~= 32 and j ~= 32 then
--This is the equation from the paper which tells us which direction the result is in
local tan = atan2(i-33,j-33)+0.25
if (tan > 1) tan = tan-1
local dInD = flr(360*tan)
--Our polar histogram has sectors ALPHA degrees wide, this rounds the direction down to a multiple of
--the sector size so we can use it as an index
local dir = dInD - dInD%ALPHA
polarHist[dir] += certaintyTable[tableIndex.left+i][tableIndex.down+j]
end
end
As a note I dont have access to the standard lua libraries.
and here is cleartable in case I am doing that wrong
function cleartable(x1,x2,y1,y2)
if x1 == x2 then
for j=y1,y2 do
certaintyTable[x1][j] = nil
end
elseif y1==y2 then
for i=x1,x2 do
certaintyTable[i][y1] = nil
end
else
for i=x1,x2 do
for j=y1,y2 do
certaintyTable[i][j] = nil
end
end
end
end
Thanks for looking at my problem