diff --git a/apps/user_ldap/lib/User/User.php b/apps/user_ldap/lib/User/User.php
index d45e19b39d61..2ebee207a65b 100644
--- a/apps/user_ldap/lib/User/User.php
+++ b/apps/user_ldap/lib/User/User.php
@@ -4,6 +4,7 @@
  * @author Joas Schilling <nickvergessen@owncloud.com>
  * @author Morris Jobke <hey@morrisjobke.de>
  * @author Thomas Müller <thomas.mueller@tmit.eu>
+ * @author Jörn Friedrich Dreyer <jfd@butonic.de>
  *
  * @copyright Copyright (c) 2016, ownCloud, Inc.
  * @license AGPL-3.0
@@ -110,6 +111,12 @@ public function __construct($username, $dn, IUserTools $access,
 		IConfig $config, FilesystemHelper $fs, Image $image,
 		LogWrapper $log, IAvatarManager $avatarManager, IUserManager $userManager) {
 
+		if ($username === null) {
+			throw new \InvalidArgumentException("uid for '$dn' must not be null!");
+		} else if ($username === '') {
+			throw new \InvalidArgumentException("uid for '$dn' must not be an empty string!");
+		}
+
 		$this->access        = $access;
 		$this->connection    = $access->getConnection();
 		$this->config        = $config;