diff --git a/lib/Db/BoardMapper.php b/lib/Db/BoardMapper.php
index 5de370094..e1b2d0598 100644
--- a/lib/Db/BoardMapper.php
+++ b/lib/Db/BoardMapper.php
@@ -176,7 +176,7 @@ class BoardMapper extends DeckMapper implements IPermissionMapper {
 					return new User($user);
 				} else {
 					\OC::$server->getLogger()->debug('User ' . $acl->getId() . ' not found when mapping acl ' . $acl->getParticipant());
-					return $participant;
+					return null;
 				}
 			}
 			if($acl->getType() === Acl::PERMISSION_TYPE_GROUP) {
@@ -185,7 +185,7 @@ class BoardMapper extends DeckMapper implements IPermissionMapper {
 					return new Group($group);
 				} else {
 					\OC::$server->getLogger()->debug('Group ' . $acl->getId() . ' not found when mapping acl ' . $acl->getParticipant());
-					return $participant;
+					return null;
 				}
 			}
 			throw new \Exception('Unknown permission type for mapping Acl');
@@ -198,7 +198,11 @@ class BoardMapper extends DeckMapper implements IPermissionMapper {
 	public function mapOwner(Board &$board) {
 		$userManager = $this->userManager;
 		$board->resolveRelation('owner', function($owner) use (&$userManager) {
-			return new User($userManager->get($owner));
+			$user = $userManager->get($owner);
+			if($user !== null) {
+				return new User($user);
+			}
+			return null;
 		});
 	}
 
diff --git a/lib/Db/CardMapper.php b/lib/Db/CardMapper.php
index 1542a95ea..e5f4fa284 100644
--- a/lib/Db/CardMapper.php
+++ b/lib/Db/CardMapper.php
@@ -31,6 +31,7 @@ use OCP\IUserManager;
 class CardMapper extends DeckMapper implements IPermissionMapper {
 
 	private $labelMapper;
+	private $userManager;
 
 	public function __construct(IDBConnection $db, LabelMapper $labelMapper, IUserManager $userManager) {
 		parent::__construct($db, 'deck_cards', '\OCA\Deck\Db\Card');
@@ -131,7 +132,11 @@ class CardMapper extends DeckMapper implements IPermissionMapper {
 	public function mapOwner(Card &$card) {
 		$userManager = $this->userManager;
 		$card->resolveRelation('owner', function($owner) use (&$userManager) {
-			return new User($userManager->get($owner));
+			$user = $userManager->get($owner);
+			if($user !== null) {
+				return new User($user);
+			}
+			return null;
 		});
 	}
 
diff --git a/lib/Db/RelationalObject.php b/lib/Db/RelationalObject.php
index e5740e754..d5a6fc18e 100644
--- a/lib/Db/RelationalObject.php
+++ b/lib/Db/RelationalObject.php
@@ -25,7 +25,8 @@ namespace OCA\Deck\Db;
 
 class RelationalObject implements \JsonSerializable {
 
-	private $primaryKey;
+	protected $primaryKey;
+	protected $object;
 
 	/**
 	 * RelationalObject constructor.