diff --git a/dbxio/blobs/download.py b/dbxio/blobs/download.py
index c0b0ae5..9d233f0 100644
--- a/dbxio/blobs/download.py
+++ b/dbxio/blobs/download.py
@@ -22,6 +22,7 @@ def download_blob_tree(
         relative_blob_path = blob.name[len(prefix_path) + 1 :] if prefix_path else blob.name
         if not relative_blob_path:
             # if the prefix path is full path to one file
+            assert object_storage_client.blobs_path, 'blobs_path is not set'
             relative_blob_path = blob.name[len(object_storage_client.blobs_path) + 1 :]
 
         if is_dir: