diff --git a/lib/SimpleSAML/XML/Validator.php b/lib/SimpleSAML/XML/Validator.php
index 8551cf99f29e60168a42440a4a766a45658feee0..f1dedd90804cce364d42fd656e97586a1a65d60f 100644
--- a/lib/SimpleSAML/XML/Validator.php
+++ b/lib/SimpleSAML/XML/Validator.php
@@ -1,5 +1,7 @@
 <?php
 
+require_once((isset($SIMPLESAML_INCPREFIX)?$SIMPLESAML_INCPREFIX:'') . 'xmlseclibs.php');
+
 /**
  * This class implements helper functions for XML validation.
  *