diff --git a/backend/address2UPRN/main.py b/backend/address2UPRN/main.py
index 4c22db44..d0ba36e6 100644
--- a/backend/address2UPRN/main.py
+++ b/backend/address2UPRN/main.py
@@ -1,3 +1,5 @@
+from typing import Optional
+
 from epc_api.client import EpcClient
 import os
 from urllib.parse import urlencode
@@ -295,7 +297,10 @@ def resolve_uprns_for_postcode_group(
 
 
 def save_results_to_s3(
-    results_df: pd.DataFrame, task_id: str, sub_task_id: str, bucket_name: str = None
+    results_df: pd.DataFrame,
+    task_id: str,
+    sub_task_id: str,
+    bucket_name: Optional[str] = None,
 ) -> bool:
     """
     Save results DataFrame to S3 as CSV.
diff --git a/backend/ordnanceSurvey/main.py b/backend/ordnanceSurvey/main.py
index 5c8373a1..70b45079 100644
--- a/backend/ordnanceSurvey/main.py
+++ b/backend/ordnanceSurvey/main.py
@@ -68,7 +68,10 @@ def get_ordance_survey_record(row, cache=None):
 
 
 def save_results_to_s3(
-    results_df: pd.DataFrame, task_id: str, sub_task_id: str, bucket_name: str = None
+    results_df: pd.DataFrame,
+    task_id: str,
+    sub_task_id: str,
+    bucket_name: Optional[str] = None,
 ) -> bool:
     """
     Save results DataFrame to S3 as CSV in a parent folder structure.