diff --git a/shared/datastore/src/put.py b/shared/datastore/src/put.py
index 3e6c99e..372a1d7 100644
--- a/shared/datastore/src/put.py
+++ b/shared/datastore/src/put.py
@@ -20,7 +20,7 @@ def put(
     assert isinstance(object_name, str)
     assert len(object_name) > 0
     # prepare for saving
-    num_bytes = buffer.tell()
+    num_bytes = len(buffer.getvalue())
     buffer.seek(0)
     # send data to bucket
     try: