diff --git a/src/junglescout/models/responses/serializer_helpers.py b/src/junglescout/models/responses/serializer_helpers.py
index 7ac74f2..b3b31b9 100644
--- a/src/junglescout/models/responses/serializer_helpers.py
+++ b/src/junglescout/models/responses/serializer_helpers.py
@@ -2,11 +2,11 @@
 from typing import Optional
 
 
-def serialize_datetime(dt: Optional[datetime]) -> str:
+def serialize_datetime(dt: Optional[datetime]) -> Optional[str]:
     """Serialize a datetime object to a datetime string."""
     return dt.strftime("%Y-%m-%dT%H:%M:%S.%f") if dt else None
 
 
-def serialize_date(dt: Optional[datetime]) -> str:
+def serialize_date(dt: Optional[datetime]) -> Optional[str]:
     """Serialize a datetime object to a date string."""
     return dt.strftime("%Y-%m-%d") if dt else None