diff --git a/modules/saml2/lib/Message.php b/modules/saml2/lib/Message.php
index 1b049fadf6a8ceea55fa56f238af642c950af9e1..10e41a4dcb59c84eeb573470c7429baca17c2e28 100644
--- a/modules/saml2/lib/Message.php
+++ b/modules/saml2/lib/Message.php
@@ -589,8 +589,7 @@ class sspmod_saml2_Message {
 		$sessionLifetime = $config->getInteger('session.duration', 8*60*60);
 		$a->setSessionNotOnOrAfter(time() + $sessionLifetime);
 
-		$sessionIndex = $session->getSessionIndex();
-		$a->setSessionIndex($sessionIndex);
+		$a->setSessionIndex(SimpleSAML_Utilities::generateID());
 
 		/* Add attributes. */