diff --git a/cuenca/resources/commissions.py b/cuenca/resources/commissions.py
index 401397b0..febdd28b 100644
--- a/cuenca/resources/commissions.py
+++ b/cuenca/resources/commissions.py
@@ -1,4 +1,4 @@
-from typing import ClassVar
+from typing import ClassVar, Optional
 
 from pydantic.dataclasses import dataclass
 
@@ -9,4 +9,4 @@
 class Commission(Transaction, Cacheable):
     _resource: ClassVar = 'commissions'
 
-    related_transaction_uri: str
+    related_transaction_uri: Optional[str]