diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index c073b4e41ce3..3b3c79b36158 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -1427,7 +1427,7 @@ async def persist_and_notify_client_event(
             # Validate a newly added alias or newly added alt_aliases.
 
             original_alias = None
-            original_alt_aliases: List[str] = []
+            original_alt_aliases: object = []
 
             original_event_id = event.unsigned.get("replaces_state")
             if original_event_id:
@@ -1455,9 +1455,7 @@ async def persist_and_notify_client_event(
             # If the old version of alt_aliases is of an unknown form,
             # completely replace it.
             if not isinstance(original_alt_aliases, (list, tuple)):
-                # type-ignore: although original_alt_aliases is defined as List[str],
-                # it may be set to `alt_aliases` from `original_event`, i.e. to any
-                # JSON value. Therefore this is not unreachable.
+                # TODO: check that the original_alt_aliases' entries are all strings
                 original_alt_aliases = []  # type: ignore[unreachable]
 
             # Check that each alias is currently valid.