diff --git a/doc/src/sgml/ref/alter_table.sgml b/doc/src/sgml/ref/alter_table.sgml
index 85705e9..0354dde 100644
--- a/doc/src/sgml/ref/alter_table.sgml
+++ b/doc/src/sgml/ref/alter_table.sgml
@@ -805,7 +805,7 @@ ALTER TABLE [ IF EXISTS ] <replaceable class="PARAMETER">name</replaceable>
       <term><replaceable class="PARAMETER">index_name</replaceable></term>
       <listitem>
        <para>
-        The index name on which the table should be marked for clustering.
+        The name of an existing index.
        </para>
       </listitem>
      </varlistentry>