diff -u b/src/Fast404.php b/src/Fast404.php
--- b/src/Fast404.php
+++ b/src/Fast404.php
@@ -119,6 +119,10 @@
       if (in_array($prefix, $lang_negotiation_url_info['prefixes'])) {
         if ($pos !== FALSE) {
           $path = substr($path, $pos);
+          if (empty($path) || $path == '/') {
+            // This path is the front page for a language prefix.
+            return;
+          }
         }
         else {
           // This path is the front page for a language prefix.