diff --git a/libraries/classes/Navigation/NavigationTree.php b/libraries/classes/Navigation/NavigationTree.php
index 0ff9726da1..78659a4b7f 100644
--- a/libraries/classes/Navigation/NavigationTree.php
+++ b/libraries/classes/Navigation/NavigationTree.php
@@ -764,7 +764,7 @@ class NavigationTree
                 }
 
                 $groups[$key] = new Node(
-                    htmlspecialchars($key),
+                    htmlspecialchars((string) $key),
                     Node::CONTAINER,
                     true
                 );