diff --git a/lib/SimpleSAML/XML/Validator.php b/lib/SimpleSAML/XML/Validator.php
index 04a1174fcdaa94aa382a45057d83014d338afadf..becf3516e74c6ca9ffc4e19619d09a6cce597e42 100644
--- a/lib/SimpleSAML/XML/Validator.php
+++ b/lib/SimpleSAML/XML/Validator.php
@@ -23,7 +23,7 @@ class Validator
     private $x509Certificate;
 
     /**
-     * @var array This variable contains the nodes which are signed.
+     * @var array|null This variable contains the nodes which are signed.
      */
     private $validNodes = null;