diff --git a/lib/SimpleSAML/SessionHandler.php b/lib/SimpleSAML/SessionHandler.php
index 5ee37605ec6c26acdc4871ec102caabfa7488619..3861aa786a1d0d0ab92e4cee5c65f374e6df090b 100644
--- a/lib/SimpleSAML/SessionHandler.php
+++ b/lib/SimpleSAML/SessionHandler.php
@@ -155,7 +155,7 @@ abstract class SessionHandler
         return [
             'lifetime' => $config->getInteger('session.cookie.lifetime', 0),
             'path'     => $config->getString('session.cookie.path', '/'),
-            'domain'   => strval($config->getString('session.cookie.domain', null)),
+            'domain'   => $config->getString('session.cookie.domain', ''),
             'secure'   => $config->getBoolean('session.cookie.secure', false),
             'samesite' => $config->getString('session.cookie.samesite', null),
             'httponly' => true,