diff --git a/src/openlayer/lib/data/_upload.py b/src/openlayer/lib/data/_upload.py
index fd0d30e9..c08f2a3f 100644
--- a/src/openlayer/lib/data/_upload.py
+++ b/src/openlayer/lib/data/_upload.py
@@ -115,6 +115,8 @@ def upload_bytes_multipart(
     if isinstance(data, bytes):
         data = io.BytesIO(data)
 
+    # fields can be None when using local storage (presigned URL has no S3 policy fields)
+    fields = fields or {}
     upload_fields = {"file": (object_name, data, content_type), **fields}
 
     encoder = MultipartEncoder(fields=upload_fields)
diff --git a/src/openlayer/lib/tracing/attachment_uploader.py b/src/openlayer/lib/tracing/attachment_uploader.py
index f7490f29..bb50448c 100644
--- a/src/openlayer/lib/tracing/attachment_uploader.py
+++ b/src/openlayer/lib/tracing/attachment_uploader.py
@@ -130,18 +130,24 @@ def upload_attachment(self, attachment: "Attachment") -> "Attachment":
             if file_bytes is None:
                 raise ValueError(f"No data available for attachment {attachment.name}")
 
-            # Upload using the shared upload function
+            # Upload using the shared upload function.
+            # presigned_response.fields is None for local storage (only S3 needs policy fields)
+            try:
+                fields = (
+                    dict(presigned_response.fields)
+                    if presigned_response.fields is not None
+                    else {}
+                )
+            except (TypeError, ValueError):
+                fields = {}
+
             upload_bytes(
                 storage=self._storage,
                 url=presigned_response.url,
                 data=file_bytes,
                 object_name=object_name,
                 content_type=attachment.media_type,
-                fields=(
-                    dict(presigned_response.fields)
-                    if presigned_response.fields
-                    else None
-                ),
+                fields=fields,
             )
 
             # Set the storage URI on the attachment