diff --git a/schema/edxml.rng b/schema/edxml.rng
index 0aba2d9f3fc50201abd39eaaa01aea450a85f9f1..cc57db4bd4ccac117d0ce02b49ef9a1d3e70e9ba 100644
--- a/schema/edxml.rng
+++ b/schema/edxml.rng
@@ -1374,6 +1374,9 @@
             <optional>
                 <attribute name="type"/>
             </optional>
+            <optional>
+                <ref name="elem.hi"/>
+            </optional>
             <text/>
         </element>
     </define>