diff --git a/lib/SimpleSAML/Auth/Simple.php b/lib/SimpleSAML/Auth/Simple.php
index d426c7ee7f200dfeb39e0e37405eb050f700d526..f3eed669ab72ec06d3101c0090e105c8d9797583 100644
--- a/lib/SimpleSAML/Auth/Simple.php
+++ b/lib/SimpleSAML/Auth/Simple.php
@@ -148,6 +148,7 @@ class SimpleSAML_Auth_Simple {
 	 *   An URL which is suitable for use in link-elements.
 	 */
 	public function getLoginURL($returnTo = NULL) {
+		assert('is_null($returnTo) || is_string($returnTo)');
 
 		if ($returnTo === NULL) {
 			$returnTo = SimpleSAML_Utilities::selfURL();