[xml] xmllint invalidation of valid xsd

["Thomas R. Jones" <thomas jones maitreyasecurity com>]

I am testing a document against a custom schema. I have checked the
schema against XMLschema.xsd as well as the W3C XSV. All seems ok.
However, when I attempt to check a structured document instance derived
from this custom schema with xmllint, I obtain the following errors:

thomas bodhisattva:~/Documents/oval/build> xmllint --noout
--schema /usr/share/xml/ovaldi/oval-definitions-schema.xsd
novell.definitions.xml.new 
/usr/share/xml/ovaldi/oval-definitions-schema.xsd:401: element
complexType: Schemas parser error : local union type: A type, derived by
list or union, must havethe simple ur-type definition as base type, not
'{http://oval.mitre.org/XMLSchema/oval-definitions-5}(NULL)'.
/usr/share/xml/ovaldi/oval-definitions-schema.xsd:410: element
complexType: Schemas parser error : local union type: A type, derived by
list or union, must havethe simple ur-type definition as base type, not
'{http://oval.mitre.org/XMLSchema/oval-definitions-5}(NULL)'.
/usr/share/xml/ovaldi/oval-definitions-schema.xsd:419: element
complexType: Schemas parser error : local union type: A type, derived by
list or union, must havethe simple ur-type definition as base type, not
'{http://oval.mitre.org/XMLSchema/oval-definitions-5}(NULL)'.
/usr/share/xml/ovaldi/oval-definitions-schema.xsd:450: element
complexType: Schemas parser error : local union type: A type, derived by
list or union, must havethe simple ur-type definition as base type, not
'{http://oval.mitre.org/XMLSchema/oval-definitions-5}(NULL)'.
/usr/share/xml/ovaldi/oval-definitions-schema.xsd:459: element
complexType: Schemas parser error : local union type: A type, derived by
list or union, must havethe simple ur-type definition as base type, not
'{http://oval.mitre.org/XMLSchema/oval-definitions-5}(NULL)'.
/usr/share/xml/ovaldi/oval-definitions-schema.xsd:468: element
complexType: Schemas parser error : local union type: A type, derived by
list or union, must havethe simple ur-type definition as base type, not
'{http://oval.mitre.org/XMLSchema/oval-definitions-5}(NULL)'.
WXS schema /usr/share/xml/ovaldi/oval-definitions-schema.xsd failed to
compile

Here is the relevant portion of the schema:
<xsd:complexType name="EntityBaseType" abstract="true">
                <xsd:simpleContent>
                        <xsd:extension base="xsd:anySimpleType">
                                <xsd:attribute name="datatype" type="oval:DatatypeEnumeration"
use="optional" default="string" />
                                <xsd:attribute name="operation" type="oval:OperationEnumeration"
use="optional" default="equals" />
                                <xsd:attribute name="var_ref" type="oval:VariableIDPattern"
use="optional" />
                        </xsd:extension>
                </xsd:simpleContent>
        </xsd:complexType>
        <xsd:complexType name="EntityObjectBaseType" abstract="true">
                <xsd:simpleContent>
                        <xsd:extension base="oval-def:EntityBaseType">
                                <xsd:attribute name="var_check" type="oval:CheckEnumeration"
use="optional" default="all" />
                        </xsd:extension>
                </xsd:simpleContent>
        </xsd:complexType>
        <xsd:complexType name="EntityObjectAnyType">
                <xsd:simpleContent>
                        <xsd:extension base="oval-def:EntityObjectBaseType" />
                </xsd:simpleContent>
        </xsd:complexType>
        <xsd:complexType name="EntityObjectBoolType">
                <xsd:simpleContent>
                        <xsd:restriction base="oval-def:EntityObjectBaseType">
                                <xsd:simpleType>
                                        <xsd:union memberTypes="xsd:boolean oval:EmptyStringType" />
                                </xsd:simpleType>
                        </xsd:restriction>
                </xsd:simpleContent>
        </xsd:complexType>

Is this a bug? If not, Where do I go with this?
Thanks for your time.

Thomas