diff --git a/lib/private/AllConfig.php b/lib/private/AllConfig.php
index 4e13d70371b7f..a58aec4aeeac2 100644
--- a/lib/private/AllConfig.php
+++ b/lib/private/AllConfig.php
@@ -358,12 +358,17 @@ public function deleteAppFromAllUsers($appName) {
 	 *     ]
 	 */
 	private function getUserValues($userId) {
-		// TODO - FIXME
-		$this->fixDIInit();
-
 		if (isset($this->userCache[$userId])) {
 			return $this->userCache[$userId];
 		}
+		if ($userId === null || $userId === '') {
+			$this->userCache[$userId]=array();
+			return $this->userCache[$userId];
+		}
+
+		// TODO - FIXME
+		$this->fixDIInit();
+
 		$data = array();
 		$query = 'SELECT `appid`, `configkey`, `configvalue` FROM `*PREFIX*preferences` WHERE `userid` = ?';
 		$result = $this->connection->executeQuery($query, array($userId));
diff --git a/lib/private/Group/Database.php b/lib/private/Group/Database.php
index e4144fdbe200e..8be24fc50de97 100644
--- a/lib/private/Group/Database.php
+++ b/lib/private/Group/Database.php
@@ -202,6 +202,11 @@ public function removeFromGroup( $uid, $gid ) {
 	 * if the user exists at all.
 	 */
 	public function getUserGroups( $uid ) {
+		//guests has empty or null $uid
+		if ($uid === null || $uid === '') {
+			return [];
+		}
+
 		$this->fixDI();
 
 		// No magic!
diff --git a/lib/private/User/Database.php b/lib/private/User/Database.php
index 69826f49be30f..a281572ad5584 100644
--- a/lib/private/User/Database.php
+++ b/lib/private/User/Database.php
@@ -75,7 +75,6 @@ class Database extends Backend implements IUserBackend {
 	 */
 	public function __construct($eventDispatcher = null) {
 		$this->cache = new CappedMemoryCache();
-		$this->cache[null] = false;
 		$this->eventDispatcher = $eventDispatcher ? $eventDispatcher : \OC::$server->getEventDispatcher();
 	}
 
@@ -239,6 +238,12 @@ public function checkPassword($uid, $password) {
 	 */
 	private function loadUser($uid) {
 		if (!isset($this->cache[$uid])) {
+			//guests $uid could be NULL or ''
+			if ($uid === null || $uid === '') {
+				$this->cache[$uid]=false;
+				return true;
+			}
+
 			$query = \OC_DB::prepare('SELECT `uid`, `displayname` FROM `*PREFIX*users` WHERE LOWER(`uid`) = LOWER(?)');
 			$result = $query->execute(array($uid));