diff --git a/schema/edxml.rng b/schema/edxml.rng
index 93388a091ce9648a45a8b090ed5bfa812755f6ec..cc5ed53d8e67b9bd9e8a15c14a89eb99f628ed8e 100644
--- a/schema/edxml.rng
+++ b/schema/edxml.rng
@@ -503,6 +503,9 @@
                     </sch:rule>
                 </sch:pattern>
             </attribute>
+            <optional>
+                <ref name="attr.xml.id"/>
+            </optional>
             <empty/>
         </element>
     </define>