diff --git a/src/php/SerializationMapper.php b/src/php/SerializationMapper.php
index c28e93135a0939197da69bf0a4da6cc3993877d3..a310632cac02d6b9611ce862c0d3e685cb5c4c44 100644
--- a/src/php/SerializationMapper.php
+++ b/src/php/SerializationMapper.php
@@ -318,7 +318,7 @@ final class SerializationMapper implements TypeFactoryInterface
     private function mapType(?APIType $type, bool $isLink = false): Type
     {
         if ($type === null) {
-            return BuiltinType::get('any');
+            return BuiltinType::get('unknown');
         }
 
         if (!$type->isCollection()) {