xmllint invalidation of valid xsd

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:

thomasbodhisattva:~/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
'{ht
tp://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
'{ht
tp://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
'{ht
tp://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
'{ht
tp://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
'{ht
tp://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
'{ht
tp://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="ovalatatypeE
numeration"
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

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