diff --git a/reference/mongo/mongocollection/ensureindex.xml b/reference/mongo/mongocollection/ensureindex.xml
index 8dfe5c2781..5924088eca 100644
--- a/reference/mongo/mongocollection/ensureindex.xml
+++ b/reference/mongo/mongocollection/ensureindex.xml
@@ -15,7 +15,11 @@
   <methodsynopsis>
    <modifier>public</modifier> <type>boolean</type><methodname>MongoCollection::ensureIndex</methodname>
    <methodparam><type>string|array</type><parameter>keys</parameter></methodparam>
+   <methodparam><type>boolean</type><parameter>unique</parameter></methodparam>
   </methodsynopsis>
+  <para>
+   A unique index cannot be created on a field if multiple existing documents do not contain the field.  The field is effectively &null; for these documents and thus already non-unique.
+  </para>
  </refsect1>
 
  <refsect1 role="parameters"> 
@@ -32,6 +36,16 @@
       </para>
      </listitem>
     </varlistentry>   
+    <varlistentry>
+     <term>
+      <parameter>unique</parameter>
+     </term>
+     <listitem>
+      <para>
+       If the index should be unique.
+      </para>
+     </listitem>
+    </varlistentry>   
    </variablelist>
   </para>
  </refsect1>
@@ -65,6 +79,9 @@ $c->ensureIndex(array('w' => -1));
 // create an index on 'z' ascending and 'zz' descending
 $c->ensureIndex(array('z' => 1, 'zz' => -1));
 
+// create a unique index on 'x'
+$c->ensureIndex(array('x' => 1), true);
+
 ?>
 ]]>
    </programlisting>