Problem in validating the scema

[Rac]

i have a piece of code:

<xs:element name="AdjustmentIndicator">
		<xs:simpleType>
			<xs:restriction base="xs:NMTOKEN">
				<xs:enumeration value="PriorOrCurrent"/>
				<xs:enumeration value="Prior"/>
				<xs:enumeration value="Current"/>
			</xs:restriction>
		</xs:simpleType>
	</xs:element>

need to pass null value in adjustment indicator.
how to do that.if i pass null or do not pass anything then it does not validate.