diff --git a/docs/schemas/domaincommon.rng b/docs/schemas/domaincommon.rng
index 4dbda6932da84268d7cc1f0f14d0b463c073c419..874af3ffa3f5211948a4b19b0866b8375db8118e 100644
--- a/docs/schemas/domaincommon.rng
+++ b/docs/schemas/domaincommon.rng
@@ -1415,9 +1415,11 @@
 
   <define name="diskBackingStore">
     <element name="backingStore">
-      <attribute name="index">
-        <ref name="positiveInteger"/>
-      </attribute>
+      <optional>
+        <attribute name="index">
+          <ref name="positiveInteger"/>
+        </attribute>
+      </optional>
       <interleave>
         <ref name="diskSource"/>
         <ref name="diskBackingChain"/>