diff --git a/bedup/platform/btrfs.py b/bedup/platform/btrfs.py
index 0ced353..73aad0c 100644
--- a/bedup/platform/btrfs.py
+++ b/bedup/platform/btrfs.py
@@ -542,7 +542,7 @@ def read_root_tree(volume_fd):
     # Deal with parent_root_id > root_id,
     # happens after moving subvolumes.
     while ri_rel:
-        for (root_id, ri) in ri_rel.items():
+        for (root_id, ri) in dict(ri_rel).items():
             if ri.parent_root_id not in root_info:
                 continue
             parent_path = root_info[ri.parent_root_id].path