lua flatten table

flatten.lua

local function noop(...)
    return ...
end

-- convert a nested table to a flat table
local function flatten(t, sep, key_modifier, res)
    if type(t) ~= 'table' then
        return t
    end

    if sep == nil then
        sep = '.'
    end

    if res == nil then
        res = {}
    end

    if key_modifier == nil then
        key_modifier = noop
    end

    for k, v in pairs(t) do
        if type(v) == 'table' then
            local v = flatten(v, sep, key_modifier, {})
            for k2, v2 in pairs(v) do
                res[key_modifier(k) .. sep .. key_modifier(k2)] = v2
            end
        else
            res[key_modifier(k)] = v
        end
    end
    return res
end

return flatten

This is not working (with lua 5.4)?
I tried:

local test1 = {{1}, 2, {3, {4}}}
local test2 = flatten(test1)

for i=1, #test2 do --Test
    print(test2[i])
end

The returned table is empty.