diff --git i/simple_sitemap.routing.yml w/simple_sitemap.routing.yml
index e03cd8b..1d35b8e 100644
--- i/simple_sitemap.routing.yml
+++ w/simple_sitemap.routing.yml
@@ -19,6 +19,8 @@ simple_sitemap.sitemap_variant:
   requirements:
     # Sitemaps are accessible for everyone.
     _access: 'TRUE'
+    # The variant parameter only accepts ASCII characters.
+    variant: '^[\x21-\x2E\x30-\x7E]+$'
 
 simple_sitemap.sitemap_xsl:
   path: '/sitemap_generator/{sitemap_generator}/sitemap.xsl'