diff --git a/data/model/legacy.py b/data/model/legacy.py
index bde1b4ad8..7ff24adc8 100644
--- a/data/model/legacy.py
+++ b/data/model/legacy.py
@@ -1459,7 +1459,8 @@ def set_image_size(docker_image_id, namespace_name, repository_name, image_size,
       # total_size = image_size + parent_image.storage.aggregate_size
       total_size = (ImageStorage.select(fn.Sum(ImageStorage.image_size))
                                 .join(Image)
-                                .where(Image.id << ancestors)) + image_size
+                                .where(Image.id << ancestors)
+                                .scalar()) + image_size
 
       image.storage.aggregate_size = total_size
     except Image.DoesNotExist: