diff --git a/src/Serializer/PartNormalizer.php b/src/Serializer/PartNormalizer.php
index 9050abfc..775df77f 100644
--- a/src/Serializer/PartNormalizer.php
+++ b/src/Serializer/PartNormalizer.php
@@ -92,7 +92,7 @@ public function normalize($object, ?string $format = null, array $context = []):
         return $data;
     }
 
-    public function supportsDenormalization($data, string $type, string $format = null, array $context = []): bool
+    public function supportsDenormalization($data, string $type, ?string $format = null, array $context = []): bool
     {
         //Only denormalize if we are doing a file import operation
         if (!($context['partdb_import'] ?? false)) {