Why is this __index metatable not saving the row correctly? [closed]

The goal of this code is to make a grid that is infinitely large by creating a new row whenever one is asked for (via the __index metamethod).

When I ask for a row from the board I do get a row, but it doesn't save it -- if I ask for the same row number, I don't get the same row table out.

local board = {};
setmetatable(board, {__index = function(row)
    local t = {};
    board[row] = t;
    return t;
end});

board[0][0] = true;

assert(board[0] == board[0], "inconsistent");
-- This assert fails, so that means board[0] is NOT the
-- same thing as board[0]
-- Why?
assert(board[0][0] == true, "(0, 0) not set");
-- (This one also fails, for the same reason, probably)

Code partially works. asserts fail, but I can't see why.

I'm obviously missing something here.