diff --git a/src/SchemaTraverser.php b/src/SchemaTraverser.php
index 2e01ab0..564a0d9 100644
--- a/src/SchemaTraverser.php
+++ b/src/SchemaTraverser.php
@@ -70,7 +70,7 @@ public function __construct(bool $assertConstraints = true, bool $ignoreUnknown
      *
      * @throws TraverserException
      */
-    public function traverse(mixed $data, SchemaInterface $schema, VisitorInterface $visitor = null): mixed
+    public function traverse(mixed $data, SchemaInterface $schema, ?VisitorInterface $visitor = null): mixed
     {
         $this->pathStack = [];