diff --git a/library/HTMLPurifier/Filter/YouTube.php b/library/HTMLPurifier/Filter/YouTube.php
index 276d8362..d86509ce 100644
--- a/library/HTMLPurifier/Filter/YouTube.php
+++ b/library/HTMLPurifier/Filter/YouTube.php
@@ -19,7 +19,7 @@ class HTMLPurifier_Filter_YouTube extends HTMLPurifier_Filter
         $pre_regex = '#<object[^>]+>.+?' .
             '(?:http:)?//www.youtube.com/((?:v|cp)/[A-Za-z0-9\-_=]+).+?</object>#s';
         $pre_replace = '<span class="youtube-embed">\1</span>';
-        return preg_replace($pre_regex, $pre_replace, $html);
+        return preg_replace($pre_regex, $pre_replace, (string)$html);
     }
 
     /**
@@ -31,7 +31,7 @@ class HTMLPurifier_Filter_YouTube extends HTMLPurifier_Filter
     public function postFilter($html, $config, $context)
     {
         $post_regex = '#<span class="youtube-embed">((?:v|cp)/[A-Za-z0-9\-_=]+)</span>#';
-        return preg_replace_callback($post_regex, array($this, 'postFilterCallback'), $html);
+        return preg_replace_callback($post_regex, array($this, 'postFilterCallback'), (string)$html);
     }
 
     /**