diff --git a/changelog.d/14171.feature b/changelog.d/14171.feature
new file mode 100644
index 000000000000..68de079cf317
--- /dev/null
+++ b/changelog.d/14171.feature
@@ -0,0 +1 @@
+Experimental support for [MSC3856](https://github.com/matrix-org/matrix-spec-proposals/pull/3856): threads list API.
diff --git a/synapse/storage/databases/main/relations.py b/synapse/storage/databases/main/relations.py
index ac9b96ab4401..7c54ce0b2e3d 100644
--- a/synapse/storage/databases/main/relations.py
+++ b/synapse/storage/databases/main/relations.py
@@ -138,7 +138,7 @@ def threads_backfill_txn(txn: LoggingTransaction) -> int:
             if isinstance(txn.database_engine, PostgresEngine):
                 txn.execute_values(sql % ("?",), rows, fetch=False)
             else:
-                txn.execute_batch(sql % ("?, ?, ?, ?, ?",), rows)
+                txn.execute_batch(sql % ("(?, ?, ?, ?, ?)",), rows)
 
             # Mark the progress.
             self.db_pool.updates._background_update_progress_txn(