diff --git a/synapse/rest/media/v1/preview_url_resource.py b/synapse/rest/media/v1/preview_url_resource.py
index e1212313e..64ad218f7 100644
--- a/synapse/rest/media/v1/preview_url_resource.py
+++ b/synapse/rest/media/v1/preview_url_resource.py
@@ -334,6 +334,7 @@ class PreviewUrlResource(Resource):
 
                 # We don't just use XPATH here as that is slow on some machines.
 
+                # We clone `tree` as we modify it.
                 cloned_tree = deepcopy(tree.find("body"))
 
                 TAGS_TO_REMOVE = ("header", "nav", "aside", "footer", "script", "style",)