WordPress SEO Yoast sitemap issue

I’m having an issue with the sitemap that is being generated by the WordPress SEO by Yoast plugin.

When I visit http://www.mysite.com/sitemap_index.xml I get the following error in my browser:

“This page contains the following errors:

error on line 5 at column 6: XML declaration allowed only at the start of the document Below is a rendering of the page up to the first error.”

Google doesn’t seem to be having a problem reading the file. Is this something that I should try to fix?

Also, I submitted http://www.mysite.com/page-sitemap.xml to Google webmaster tools awhile back. Should I remove that one and use sitemap_index.xml.

Thanks