diff --git a/product_harmonized_system/models/product_category.py b/product_harmonized_system/models/product_category.py
index 6f6233e..2da1e33 100644
--- a/product_harmonized_system/models/product_category.py
+++ b/product_harmonized_system/models/product_category.py
@@ -26,5 +26,5 @@ class ProductCategory(models.Model):
         elif self.parent_id:
             res = self.parent_id.get_hs_code_recursively()
         else:
-            res = None
+            res = self.env['hs.code']
         return res