Why is my xml doc considered an element?

I am trying to load an xml file into an xmlDocument but receive an error that it cannot cast the xmlelement to a xmldocument why?

XML

<VR>
  <SubscriberID>xxxx</SubscriberID>
  <EmailAddress>m@gmail.com</EmailAddress>
  <FirstName>m</FirstName>
  <LastName>x</LastName>
  <State>CO</State>
  <Country/>
  <BirthDate>11/16/3004</BirthDate>
  <SendEmail>False</SendEmail>
  <Preference Value="true" Key="life"/>
  <Preference Value="true" Key="yo"/>
</VR>

C#

        preferenceHelper target = new preferenceHelper(); // TODO: Initialize to an appropriate value
        XmlDocument docIn = new XmlDocument();
        docIn.Load(@"C:/INTG/trunk/src/VRI.Integration.Email/Test/xmlIn.xml");
        XmlDocument expected = null; // I know this will fail, but it should compile, right?
        XmlDocument actual;
        actual = target.preferencesXmlDoc(docIn);
        Assert.AreEqual(expected, actual);
        Assert.Inconclusive("Verify the correctness of this test method.");

Error

Test method Test.preferenceHelperTest.preferencesXmlDocTest threw exception: 
System.InvalidCastException: Unable to cast object of type 'System.Xml.XmlElement' to type 'System.Xml.XmlDocument'.

I will not be adding a namespace to the xmlIn, if this is required - how might I load in my xml File?

Where it fails: actual = target.preferencesXmlDoc(docIn);

Thanks