schema appears to be invalid?

I am attempting to use the schema for xforms, but it appears to be
invalid.  Is this a bug?  Am I missing something?  I am using xml spy.
 
Unexpected child element: 'xsd:union'
 
 
<xsd:attribute name="includenamespaceprefixes" use="optional">
    <xsd:simpleType>
        <xsd:list>
            <xsd:union>
                <xsd:simpleType>
                    <xsd:restriction base="xsd:NCNAME"/>
                </xsd:simpleType>
                <xsd:simpleType>
                    <xsd:restriction base="xsd:string">
                        <xsd:enumeration value="#default"/>
                    </xsd:restriction>
                </xsd:simpleType>
            </xsd:union>
        </xsd:list>
    </xsd:simpleType>
</xsd:attribute>
 
 
Kevin Ross