diff --git a/changelogs/application_service/newsfragments/1541.feature b/changelogs/application_service/newsfragments/1541.feature
new file mode 100644
index 00000000..ee222fb0
--- /dev/null
+++ b/changelogs/application_service/newsfragments/1541.feature
@@ -0,0 +1 @@
+Add homeserver->appservice ping mechanism, as per [MSC2659](https://github.com/matrix-org/matrix-spec-proposals/pull/2659). Contributed by @tulir at @beeper.
diff --git a/data/api/client-server/appservice_ping.yaml b/data/api/client-server/appservice_ping.yaml
index 4075e901..13b7289f 100644
--- a/data/api/client-server/appservice_ping.yaml
+++ b/data/api/client-server/appservice_ping.yaml
@@ -82,6 +82,8 @@ paths:
                     The duration in milliseconds that the
                     [`/_matrix/app/v1/ping`](#post_matrixappv1ping)
                     request took from the homeserver's point of view.
+            required:
+              - duration_ms
           examples:
             application/json: {"duration_ms": 123}
         400: