diff --git i/src/PathProcessor/SitemapPathProcessor.php w/src/PathProcessor/SitemapPathProcessor.php
index b15e455..dedaf00 100644
--- i/src/PathProcessor/SitemapPathProcessor.php
+++ w/src/PathProcessor/SitemapPathProcessor.php
@@ -5,6 +5,7 @@ namespace Drupal\simple_sitemap\PathProcessor;
 use Drupal\Core\PathProcessor\InboundPathProcessorInterface;
 use Drupal\Core\PathProcessor\OutboundPathProcessorInterface;
 use Drupal\Core\Render\BubbleableMetadata;
+use Drupal\simple_sitemap\Entity\SimpleSitemap;
 use Symfony\Component\HttpFoundation\Request;
 
 /**
@@ -17,7 +18,10 @@ class SitemapPathProcessor implements InboundPathProcessorInterface, OutboundPat
    */
   public function processInbound($path, Request $request) {
     $args = explode('/', $path ?? '');
-    if (count($args) === 3 && $args[2] === 'sitemap.xml') {
+    if (count($args) === 3 && $args[2] === 'sitemap.xml'
+    // Ensure variant is ASCII - see https://www.drupal.org/project/simple_sitemap/issues/3554196.
+    && mb_check_encoding($args[1], 'ASCII')
+    && SimpleSitemap::load($args[1])) {
       $path = '/sitemaps/' . $args[1] . '/sitemap.xml';
     }