diff --git a/doc/Core.xml b/doc/Core.xml
index 8d8bc1174..01e1b03a3 100644
--- a/doc/Core.xml
+++ b/doc/Core.xml
@@ -2117,7 +2117,7 @@ DATE: when response was generated
                     <para>Optional Action Not Implemented</para>
                   </entry>
                   <entry>
-                    <para>The requested action is optional and is not implemented by the device.</para>
+                    <para>The requested action is optional and is not implemented by the device. Note that this fault may be caused by both Sender and Receiver.</para>
                   </entry>
                 </row>
                 <row>