--- a/xml/schema/prov.xsd	Tue Aug 28 15:56:36 2012 -0400
+++ b/xml/schema/prov.xsd	Wed Aug 29 08:19:06 2012 -0400
@@ -247,7 +247,7 @@
     <xs:sequence>
       <xs:element name="subordinate" type="prov:AgentRef"/>
       <xs:element name="responsible" type="prov:AgentRef"/>
-      <xs:element name="activity"    type="prov:ActivityRef"/>
+      <xs:element name="activity"    type="prov:ActivityRef" minOccurs="0"/>
       <xs:element ref="prov:label" minOccurs="0"  maxOccurs="unbounded"/> 
       <xs:element ref="prov:type"  minOccurs="0" maxOccurs="unbounded"/>
       <xs:any namespace="##other"  minOccurs="0" maxOccurs="unbounded"/>