error in generated sitemap.xml - Google Webmaster Tool and browser check

Hi guys,
I get an error when testing the generated sitemap.xml inside Google Webmaster Tool (see attachment) and also when opening the file inside a browser.
Error in browser:

This page contains the following errors:
error on line 2 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.

line 2 in sitemap.xml shows this:
<urlset xmlns:xsi='' xsi:schemaLocation='; xmlns='' >

Thanks for your help.