diff --git a/doc/src/sgml/pgupgrade.sgml b/doc/src/sgml/pgupgrade.sgml
index 30f45441978a978b3dd8eabac628d47cc71c9811..cffcebb30707c2b2954192e427aa45ab96d02184 100644
--- a/doc/src/sgml/pgupgrade.sgml
+++ b/doc/src/sgml/pgupgrade.sgml
@@ -517,6 +517,12 @@ psql --username postgres --file script.sql postgres
    <literal>-d /real-data-directory -o '-D /configuration-directory'</>.
   </para>
 
+  <para>
+   A Log-Shipping Standby Server (<xref linkend="warm-standby">) cannot
+   be upgraded because the server must allow writes.  The simplest way
+   is to upgrade the primary and use rsync to rebuild the standbys.
+  </para>
+
   <para>
    If you want to use link mode and you do not want your old cluster
    to be modified when the new cluster is started, make a copy of the