diff --git a/xep-0410.xml b/xep-0410.xml
index 24f5c6d8..71843f34 100644
--- a/xep-0410.xml
+++ b/xep-0410.xml
@@ -180,9 +180,14 @@
     <p>Therefore, a MUC service supporting this protocol may directly respond
       to a participant's Ping request to the participant's own nickname, as
       opposed to routing it to any of the participant's clients. A service
-      implementing this behavior needs to advertise the
+      implementing this optimization needs to advertise the
       <tt>self-ping-optimization</tt> feature in the &xep0030; response on
-      the individual MUC room JIDs.</p>
+      the individual MUC room JIDs, and it MUST respond to a self-ping request
+      as follows:</p>
+    <ul>
+      <li><strong>Successful IQ response</strong>: the client is joined to the MUC.</li>
+      <li><strong>Error (&lt;not-acceptable&gt;)</strong>: the client is not joined to the MUC.</li>
+    </ul>
 <example caption='MUC Service Advertises Self-Ping Optimization'><![CDATA[
 <iq from='darkcave@chat.shakespeare.lit'
     type='result'>