diff --git a/lib/SimpleSAML/Auth/Source.php b/lib/SimpleSAML/Auth/Source.php
index b47f53aff44586c2d93d8aedf01f6f3f7ec1a7db..3058e810197b1d6e01736661d98cdadd8e4ce4a8 100644
--- a/lib/SimpleSAML/Auth/Source.php
+++ b/lib/SimpleSAML/Auth/Source.php
@@ -162,7 +162,6 @@ abstract class SimpleSAML_Auth_Source
      */
     public function initLogin($return, $errorURL = null, array $params = array())
     {
-        assert('is_string($authId)');
         assert('is_string($return) || is_array($return)');
         assert('is_string($errorURL) || is_null($errorURL)');