diff --git a/core/search/service.py b/core/search/service.py
index aad21dc2e8..8efb159a51 100644
--- a/core/search/service.py
+++ b/core/search/service.py
@@ -168,8 +168,9 @@ def __init__(self, client: OpenSearch, base_revision_name: str):
         self._indexes_created: List[str] = []
 
         # Documents are not allowed to automatically create indexes.
+        # AWS OpenSearch only accepts the "flat" format
         self._client.cluster.put_settings(
-            body={"persistent": {"action": {"auto_create_index": "false"}}}
+            body={"persistent": {"action.auto_create_index": "false"}}
         )
 
     def indexes_created(self) -> List[str]: