Is it correct schema definition?

Hi,

The following piece of schema is from a released Schema definition.The
problem is the attribute "fixed". In the element definition, fixed="" is
correct or not?

  <complexType name="countryItemType">
    <simpleContent>
      <restriction base="stringItemType">
        <length value="0" fixed="true" />
      </restriction>
    </simpleContent>
  </complexType>

  <element id="id_Afghanistan" name="Afghanistan" type="countryItemType"
substitutionGroup="CountryItemHead" fixed="" nillable="true" />

Thanks,
John