diff --git a/external/billing/schemas.py b/external/billing/schemas.py
index 002238c..5ea9c92 100644
--- a/external/billing/schemas.py
+++ b/external/billing/schemas.py
@@ -62,7 +62,7 @@ class NotifyReceivedBillRequestSchema(BaseSchema):
 
 class CreateBillingResponseSchema(BaseSchema):
     ok: bool
-    message:Optional[str]
+    message:Optional[str] = None
 
 class NotifyReceivedBillResponseSchema(BaseSchema):
     ok: bool