xpath_string and group by

Hi all,

Using 8.1.4 and contrib/xml2. When I do a

select xpath_string(note, '//Thing') as note,
count(aDate) from theTable
group by lower(xpath_string(note, '//Thing'))
order by 2 desc;

I get an error:
GROUP BY must contain note.
But I can do that for a plain text/varchar field. Adding the non-xpath
note field messes up the grouping.

Can someone explain what interaction of features causes the above? I'm
finding that to do the counts the way I want (case-insensitive with
trimmed blanks) I end up doing some variation of:

select note, count(aDate) from
(select lower(xpath_string(note, '//Thing')) as note, aDate from
theTable) as foo
group by note

tia,
arturo