diff --git a/lib/SimpleSAML/Auth/Source.php b/lib/SimpleSAML/Auth/Source.php
index 358cd178d720be82c81bce349178dc1eb326b8e2..802035e4e2903e76a1a2f854a275bd3f075611ec 100644
--- a/lib/SimpleSAML/Auth/Source.php
+++ b/lib/SimpleSAML/Auth/Source.php
@@ -56,7 +56,7 @@ abstract class SimpleSAML_Auth_Source {
 			$source = $config->getArray($id);
 
 			if (!array_key_exists(0, $source) || !is_string($source[0])) {
-				throw new Exception('Invalid authentication source \'' . $authId .
+				throw new Exception('Invalid authentication source \'' . $id .
 					'\': First element must be a string which identifies the authentication source.');
 			}