[Plugin: WordPress SEO by Yoast] XML Parsing Error: no element found Line Number 1, Column 1:

WordPress version: 3.4.1
WordPress SEO version: 1.2.5
WP Minify: 1.2.0

On finding sitemaps not working as expected I have tracked it down to if WP Minify is active the post sitemap returns:

XML Parsing Error: no element found
Location: http://plantnetwork.org/post-sitemap.xml
Line Number 1, Column 1:

page-sitemap.xml and pn_directory-sitemap.xml work as expected from:
http://plantnetwork.org/sitemap_index.xml

I went back to having the default Twenty Eleven theme with only WordPress SEO and WP Minify active.

Any ideas?

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