diff --git a/lib/SimpleSAML/SessionHandler.php b/lib/SimpleSAML/SessionHandler.php
index 4fdcb6dcff2922b9325d7341bf575a8988e2be00..7fcf4b95e9946fdb7cecfc305768b842739b32e1 100644
--- a/lib/SimpleSAML/SessionHandler.php
+++ b/lib/SimpleSAML/SessionHandler.php
@@ -159,7 +159,7 @@ abstract class SessionHandler
         return [
             'lifetime' => $config->getInteger('session.cookie.lifetime', 0),
             'path'     => $config->getString('session.cookie.path', '/'),
-            'domain'   => $config->getString('session.cookie.domain', null),
+            'domain'   => strval($config->getString('session.cookie.domain', null)),
             'secure'   => $config->getBoolean('session.cookie.secure', false),
             'samesite' => $config->getString('session.cookie.samesite', null),
             'httponly' => true,