diff --git a/src/cfdppy/handler/source.py b/src/cfdppy/handler/source.py
index cc4a238..da02f7d 100644
--- a/src/cfdppy/handler/source.py
+++ b/src/cfdppy/handler/source.py
@@ -292,7 +292,7 @@ def progress(self) -> int:
         return self._params.fp.progress
 
     @property
-    def file_size(self) -> int:
+    def file_size(self) -> Optional[int]:
         return self._params.fp.file_size
 
     def get_put_request(self) -> Optional[PutRequest]: