diff --git a/sitemap_generator/util.py b/sitemap_generator/util.py
index b6354a0..6dccc79 100644
--- a/sitemap_generator/util.py
+++ b/sitemap_generator/util.py
@@ -59,6 +59,7 @@
     <lastmod>{}</lastmod>
 </url>
 '''
+ET.register_namespace('', 'http://www.sitemaps.org/schemas/sitemap/0.9')
 
 
 def write_tree(tree, file):