diff --git a/data/event-schemas/schema/m.image_pack.rooms.yaml b/data/event-schemas/schema/m.image_pack.rooms.yaml
index 20e70b6d..462d324e 100644
--- a/data/event-schemas/schema/m.image_pack.rooms.yaml
+++ b/data/event-schemas/schema/m.image_pack.rooms.yaml
@@ -31,6 +31,10 @@ properties:
           pack. Clients MAY present appropriate UI to handle this case.
         type: object
         additionalProperties:
+          description: |
+            Map from image pack `state_key` to an empty object.
+            
+            The bottom-level object is reserved ...
           type: object
           additionalProperties:
             type: object