ERROR: XML Sitemap VERSUS pinging Yahoo! & Ask.com

Seems to me like a bug:

I have XML-Sitemap function activated & it’s available via /sitemap_index.xml – all seems good!

BUT, if I am going to check additionally Yahoo! & Ask.com, all out of sudden the /sitemap_index.xml ends up in a 404!!!

http://wordpress.org/extend/plugins/wordpress-seo/