Undefined index: mod in /wordpress-seo/inc/class-sitemaps.php on line 1226

+1 I am having the same problem as well.

+2 This error/warning popped out 4 times in just 20 minutes.

PHP Notice: Undefined index: mod in /home/XXX/public_html/XXX/wp-content/plugins/wordpress-seo/inc/class-sitemaps.php on line 1226

They’ve got a patch for it. Maybe it will go away in next, next next, in a few updates from now.

https://github.com/Yoast/wordpress-seo/issues/2245

Same here, can not get sitemaps to work on a new site…investigation shows this error when trying to open the file.
When testing the sitemap in Webmastertools I get “unsupported format”