Function in register_sitemap not getting called

Adapting this code sample, I started with this code in my functions.php file:

function my_make_mycustom_sitemap() {
/* some stuff here, including logging to a file */
}

function init_do_my_sitemaps(){
	global $wpseo_sitemaps;
	if (method_exists($wpseo_sitemaps, 'register_sitemap')) {
		$wpseo_sitemaps->register_sitemap('mycustom-sitemap', 'my_make_mycustom_sitemap');
	}
}

add_action('init', 'init_do_my_sitemaps', 9999);

I’m logging to a file in both functions. I can see that the register_sitemap method is executing. However, the callback function named, my_make_mycustom_sitemap, never gets called. None of its logging ever gets written.

Any help would be greatly appreciated. Thanks!