-updated_time in the global_sort_key throws exception

[manojmohangit]

I was trying to get the contact which has been updated recently.
Why /dev/api/contact?page_size=10&global_sort_key=-updated_time throws exception {"exception message":"com.google.appengine.api.datastore.DatastoreNeedIndexException: no matching index found. recommended index is:\n- kind: Contact\n properties:\n - name: type\n - name: updated_time\n direction: desc\n\nThe suggested index for this query is:\n \n \n \n <\/datastore-index>\n\n","status":"500"} ? and also what all properties are supported in global_sort_key?