Using XPath for Identity Constraint of Elements and Attributes

Hi to all, I have an XML schema document that looks like this :

<?xml version="1.0" ?>
<xs:schema id="Employees" targetNamespace="http://tempuri.org/Employee.xsd" xmlns:mstns="http://tempuri.org/Employee.xsd"
  xmlns="http://tempuri.org/Employee.xsd" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata"
  attributeFormDefault="qualified" elementFormDefault="qualified">
  <xs:element name="Employees" msdata:IsDataSet="true" msdata:EnforceConstraints="False">
    <xs:complexType>
      <xs:choice maxOccurs="unbounded">
        <xs:element name="Employee">
          <xs:complexType>
            <xs:sequence>
              <xs:element name="Firstname" type="xs:string" minOccurs="1" msdata:Ordinal="0" maxOccurs="1" />
              <xs:element name="Lastname" type="xs:string" minOccurs="1" msdata:Ordinal="1" maxOccurs="1" />
            </xs:sequence>
            <xs:attribute name="empId" form="unqualified" type="xs:byte" use="required" />
          </xs:complexType>
          <xs:key name="EmployeeKey">
            <xs:selector xpath="Employee" />
            <xs:field xpath="Employee/@empId" />
          </xs:key>
        </xs:element>
      </xs:choice>
    </xs:complexType>
  </xs:element>
</xs:schema>

I want to test whether the schema that I created validates against an instance document. Below is my sample instance document.

<?xml version="1.0" encoding="utf-8"?>
<Employees xmlns="http://tempuri.org/Employee.xsd">
  <Employee empId="1">
    <Firstname>John</Firstname>
    <Lastname>Doe</Lastname>
  </Employee>
  <Employee empId="1">
    <Firstname>Jane</Firstname>
    <Lastname>Doe</Lastname>
  </Employee>
</Employees>

As we should expect, the instance document will be invalidated since both <Employee> elements have the same empId attribute (empId="1"). When I validate this instance document against the schema that I created, it is recognized as a valid XML document. What could be the problem? Is there a syntax error in my XPath declaration?