Sitemap generated for NextGen Images throw 404 error

Hi,

Following is the sitemap generated by WordPress SEO for my webiste: http://factly.in/sitemap_index.xml

Last link in the sitemap, http://factly.in/ngg_tag-sitemap.xml is generated for NextGen Gallery Images and is throwing up a 404 error. I can remove the generated sitemap by excluding Picture tag (ngg_tag), but I don’t want to do that.

I want the images to show up in the sitemap so it is indexed on Google. How do I make this work? Any help is appreciated.

Thanks,
Shashi

https://wordpress.org/plugins/wordpress-seo/