minetest VoxelManip tomfoolery

minetest.register_node("est:bedrock", {
    tiles = {"bedrock.png"}
})

local cid_bedrock = minetest.get_content_id("est:bedrock")

local perlinconf_test = {
    offset = 0,
    scale = 1,
    spread = {x = 16, y = 16, z = 16},
    seed = 1,
    octaves = 3,
    persistence = 0.5,
    lacunarity = 2.0,
    flags = "noeased"
}

--proof that PerlinNoise and PerlinNoiseMap work outside of a mapgen thread
local perlin_test = PerlinNoise(perlinconf_test):get_2d({x = 0, y = 0})
minetest.log(tostring(perlin_test))

local function generate(minp, maxp)
    local sidelen = {
        x = maxp.x - minp.x + 1,
        y = maxp.x - minp.x + 1,
        z = maxp.x - minp.x + 1
    }
    local perlinmap_test = PerlinNoiseMap(perlinconf_test, {x = sidelen.x, y = sidelen.y, z = sidelen.z}):get_2d_map_flat(minp)

    --the problem function, returns nil when used outside of registered_on_generateds
    local vm = minetest.get_voxel_manip()
    --proof that this returns nil: the first usage, as called outside of a mapgen function, simply logs "nil"
    minetest.log(tostring(vm))
    if vm then
        local emin, emax = vm:read_from_map(minp, maxp)
        local area = VoxelArea:new{MinEdge = emin, MaxEdge = emax}
        local data = vm:get_data()

        local i
        for i in area:iterp(minp, maxp) do
            if area:position(i).y < 0 then
                data[i] = cid_bedrock
            end
        end

        vm:set_data(data)
        vm:calc_lighting()
        vm:write_to_map()
        vm:update_liquids()
    end
end

--does not work as expected!?
generate({x = -8, y = -8, z = -8}, {x = 8, y = 8, z = 8})

--does work as expected!?
table.insert(minetest.registered_on_generateds, 1, generate)