diff --git a/swh/storage/replay.py b/swh/storage/replay.py
--- a/swh/storage/replay.py
+++ b/swh/storage/replay.py
@@ -66,6 +66,10 @@
 
 
 OBJECT_FIXERS = {
+    # drop the metadata field from the revision (if any); this field is
+    # about to be dropped from the data model (in favor of
+    # raw_extrinsic_metadata) and there can be bogus values in the existing
+    # journal (metadata with \0000 in it)
     "revision": partial(remove_keys, keys=("metadata",)),
 }