GML and nullable Foreign keys

In my xsd Schema exists foreign key who is nullable. When I create GML related to that Schema, Validator says:

Cvc-identity-constraint.4.3: Key 'foreignKey_admKucniBroj_zgrada' With Value 'null' Not Found For Identity Constraint Of Element 'KAT_CadastreFeatureCollection'.. Line '57363', Column '37'.

FK from Schema:

    <xsd:keyref name="foreignKey_admKucniBroj_zgrada" refer="fgu:primaryKey_katZgrada">
        <xsd:selector xpath=".//fgu:ADM_Kucni_Broj"/>
        <xsd:field xpath="fgu:zgrada"/>
    </xsd:keyref>

And part of Object's definition:

<xsd:complexType name="ADM_KucniBrojType" final="#all">
    <xsd:complexContent>
        <xsd:extension base="fgu:KatastarskiGeoObjektType">
            <xsd:sequence>
                <xsd:element name="zgrada" type="fgu:oidType" nillable="true"/>
            </xsd:sequence>
        </xsd:extension>
    </xsd:complexContent>
</xsd:complexType>

Does anyone knows what's hapening, what's wrong?