diff --git a/api/client-server/filter.yaml b/api/client-server/filter.yaml
index ac8e04f5..68db7a94 100644
--- a/api/client-server/filter.yaml
+++ b/api/client-server/filter.yaml
@@ -147,5 +147,7 @@ paths:
             type: object
             allOf:
               - $ref: "definitions/sync_filter.yaml"
+        404:
+          description: "Unknown filter."
       tags:
         - Room participation