XmlObject.validate() validates fine when missing attribute declared with use="required"

I have defined the following element type:
<xs:complexType name="displayName">
<xs:complexContent>
<xs:extension base="name">
<xs:attribute name="index" type="xs:boolean" use="required">
<xs:annotation>
<xs:documentation>Boolean , Indicates if the element should be indexed for search.</xs:documentation>
</xs:annotation>
</xs:attribute>
</xs:extension>
</xs:complexContent>
</xs:complexType>

When creating an element of this type it validates fine using XmlObject.validate() - even if the index attribute is missing from the element.