In REPORT DAV:principal-property-search verify the xml-namespace for the searched properties

<principal-property-search xmlns="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav">

<property-search>
  <prop>

<!-- this one has to fail --> <c:displayname/>

  </prop>
  <match>f</match>
</property-search>
<prop>
  <displayname/>
</prop>

</principal-property-search>