diff --git a/lib/private/app/appmanager.php b/lib/private/app/appmanager.php
index f826c8ba0c..8ae93f9883 100644
--- a/lib/private/app/appmanager.php
+++ b/lib/private/app/appmanager.php
@@ -148,6 +148,13 @@ class AppManager implements IAppManager {
 			return false;
 		} else {
 			$groupIds = json_decode($enabled);
+
+			if (!is_array($groupIds)) {
+				$jsonError = json_last_error();
+				\OC::$server->getLogger()->warning('AppManger::checkAppForUser - can\'t decode group IDs: ' . print_r($enabled, true) . ' - json error code: ' . $jsonError, ['app' => 'lib']);
+				return false;
+			}
+
 			$userGroups = $this->groupManager->getUserGroupIds($user);
 			foreach ($userGroups as $groupId) {
 				if (array_search($groupId, $groupIds) !== false) {