diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index d4d1fe45cc..4ec13f3311 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -9501,9 +9501,11 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </term>
       <listitem>
        <para>
-        Allows modification of the structure of system tables.
-        This is used by <command>initdb</command>.
-        This parameter can only be set at server start.
+        Allows modification of the structure of system tables as well as
+        certain other risky actions on system tables.  This is otherwise not
+        allowed even for superusers.  Ill-advised use of this setting can
+        cause irretrievable data loss or seriously corrupt the database
+        system.  Only superusers can change this setting.
        </para>
       </listitem>
      </varlistentry>
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index ba4edde71a..5fccc9683e 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -1777,7 +1777,7 @@ static struct config_bool ConfigureNamesBool[] =
 	},
 
 	{
-		{"allow_system_table_mods", PGC_POSTMASTER, DEVELOPER_OPTIONS,
+		{"allow_system_table_mods", PGC_SUSET, DEVELOPER_OPTIONS,
 			gettext_noop("Allows modifications of the structure of system tables."),
 			NULL,
 			GUC_NOT_IN_SAMPLE