LOWER function in ORDER BY and GROUP BY clause do not work with XWQL

{{velocity}}
#set($hql = "SELECT prop.value FROM XWikiDocument doc, BaseObject obj, StringProperty prop WHERE doc.fullName = obj.name AND obj.className='XWiki.XWikiUsers' AND prop.id=obj.id AND prop.name='last_name' ORDER BY lower(prop.value)")
#set($rs = $services.query.hql($hql).setLimit(10).execute())
$rs
{{/velocity}}

With XWQL, it does not work:
{{velocity}}
#set($xwql = "SELECT obj.last_name FROM Document doc, doc.object(XWiki.XWikiUsers) obj ORDER BY lower(obj.last_name)")
#set($rs = $services.query.xwql($xwql).setLimit(10).execute())
$rs
{{/velocity}}

Caused by: org.xwiki.query.jpql.parser.ParserException: [1,83] expecting: id
	at org.xwiki.query.jpql.parser.Parser.parse(Parser.java:1247)