Unique constraint in XML Schema

Let's say I have following XML file:

<authors>
   <author>a1</author>
   <author>a2</author>
   <lastmodified>2010</lastmodified>
</authors>

and an XML schema fragment:

<xs:element name="authors" maxOccurs="1">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="author" maxOccurs="unbounded" type="xs:string"> </xs:element>
      <xs:element name="lastmodified" type="xs:date" minOccurs="0"/>
    </xs:sequence>
  </xs:complexType>
  <xs:unique name="uniqueAuthor">
     <xs:selector xpath="."/>
     <xs:field xpath="author"/>
  </xs:unique>
</xs:element>

What I want is to make a constraint that will not allow two identical author values, but the one above doesn't work that way. What am I doing wrong?