Index: wp-includes/SimplePie/Sanitize.php
===================================================================
--- wp-includes/SimplePie/Sanitize.php	(revision 22945)
+++ wp-includes/SimplePie/Sanitize.php	(working copy)
@@ -246,7 +246,10 @@
 
 			if ($type & (SIMPLEPIE_CONSTRUCT_HTML | SIMPLEPIE_CONSTRUCT_XHTML))
 			{
-
+				if ( ! class_exists( 'DOMDocument' ) )
+				{
+					return '';
+				}
 				$document = new DOMDocument();
 				$document->encoding = 'UTF-8';
 				$data = $this->preprocess($data, $type);