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.