set post thumb if missing on post save (wordpress)

function init_set_thumb() {
    add_action('save_post', 'set_post_thumb', 10);
}
add_action('admin_init', 'init_set_thumb');

function set_post_thumb($post_id) {
    if(!wp_is_post_revision($post_id)) {
        $post_type = get_post_type($post_id);
        $post_thumb = get_post_thumbnail_id($post_id);
        if($post_type == 'post' && !$post_thumb) { // if post type = post and get_post_thumbnail == false
            $defaultimage = 1; // set to id of attachment to use as default post thumb
            set_post_thumbnail($post_id,$defaultimage);
        }
    }
}