diff --git a/modules/saml/www/sp/saml2-acs.php b/modules/saml/www/sp/saml2-acs.php
index 6f1266d8d63c98dfbaceb3f845e2b39256b594a2..5379676e521b3ff6ae17d615b3d75a97048a48ff 100644
--- a/modules/saml/www/sp/saml2-acs.php
+++ b/modules/saml/www/sp/saml2-acs.php
@@ -56,7 +56,7 @@ try {
 
 /* Check for duplicate assertion (replay attack). */
 $store = SimpleSAML_Store::getInstance();
-if ($store !== NULL) {
+if ($store !== FALSE) {
 	$aID = $assertion->getId();
 	if ($store->get('saml.AssertionReceived', $aID) !== NULL) {
 		$e = new SimpleSAML_Error_Exception('Received duplicate assertion.');