diff --git a/api/server-server/definitions/event-schemas/m.direct_to_device.yaml b/api/server-server/definitions/event-schemas/m.direct_to_device.yaml
index 94d446c7f..58143a37d 100644
--- a/api/server-server/definitions/event-schemas/m.direct_to_device.yaml
+++ b/api/server-server/definitions/event-schemas/m.direct_to_device.yaml
@@ -52,10 +52,9 @@ allOf:
               a map of user IDs to a map of device IDs to message bodies.
               The device ID may also be ``*``, meaning all known devices for the user.
             additionalProperties:
-              type:
-                - string
-                - string
-                - object
+              type: string
+              additionalProperties:
+                  type: string
             example: {
               "alice@example.org": {
                 "IWHQUZUIAH": {