diff --git a/library/HTMLPurifier/URISchemeRegistry.php b/library/HTMLPurifier/URISchemeRegistry.php
index 4ac8a0b7..1e3229ae 100644
--- a/library/HTMLPurifier/URISchemeRegistry.php
+++ b/library/HTMLPurifier/URISchemeRegistry.php
@@ -40,6 +40,12 @@ public static function instance($prototype = null)
      */
     public function getScheme($scheme, $config, $context)
     {
+        // Prevent deprecated "null used as array offset" in PHP 8.1+.
+        // A null scheme indicates "no scheme", so return early.
+        if ($scheme === null) {
+            return;
+        }
+        
         if (!$config) {
             $config = HTMLPurifier_Config::createDefault();
         }