add type check

Author: bytestream

library/HTMLPurifier/HTML5Config.php

@@ -64,6 +64,12 @@ public static function inherit(HTMLPurifier_Config $config)
      */
     public function __construct(HTMLPurifier_ConfigSchema $schema, $parent = null)
     {
+        if ($parent && !$parent instanceof HTMLPurifier_PropertyList) {
+            throw new InvalidArgumentException(
+                'Argument #2 ($parent) passed to ' . __METHOD__ . '() must be an instance of HTMLPurifier_PropertyList or null'
+            );
+        }
+
         // ensure 'HTML5' is among allowed 'HTML.Doctype' values
         $doctypeConfig = $schema->info['HTML.Doctype'];