Growth Problem in Minetest Mod

minetest.register_abm({
    nodenames = {"default:dirt"},
    interval = 5,
    chance = 1,
    action = function(pos)
        if minetest.get_node({pos.x, pos.y + 1, pos.z}).name == "default:air" then
            minetest.add_node({pos.x, pos.y + 1, pos.z}, {name="realism:grass"})
        end
    end,
})