diff --git a/src/PathProcessor/SitemapPathProcessor.php b/src/PathProcessor/SitemapPathProcessor.php
index 882181e..49c7d3a 100644
--- a/src/PathProcessor/SitemapPathProcessor.php
+++ b/src/PathProcessor/SitemapPathProcessor.php
@@ -18,7 +18,10 @@ class SitemapPathProcessor implements InboundPathProcessorInterface, OutboundPat
    */
   public function processInbound($path, Request $request) {
     $args = explode('/', $path ?? '');
-    if (count($args) === 3 && $args[2] === 'sitemap.xml' && SimpleSitemap::load($args[1])) {
+    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';
     }