diff --git a/vip_spec.xsd b/vip_spec.xsd
index 90953fe..f91b327 100644
--- a/vip_spec.xsd
+++ b/vip_spec.xsd
@@ -386,7 +386,7 @@
       <xs:element name="FeatureIdentifier" minOccurs="1" maxOccurs="unbounded">
         <xs:complexType>
           <xs:sequence>
-            <xs:element name="Index" type="xs:string" />
+            <xs:element name="Index" type="xs:int" />
           </xs:sequence>
         </xs:complexType>
       </xs:element>