Bug? Changing where distinct occurs produces error?

OK I'm either insane or found a bug in 8.1.3

If you execute say:

SELECT DISTINCT(ua.user_id),pa.poll_id FROM user_answers ua, poll_answers
pa WHERE pa.poll_answer_id = ua.poll_answer_id AND ua.active='Y';

Everything is fine, however if you run

SELECT pa.poll_id,DISTINCT(ua.user_id) FROM user_answers ua, poll_answers
pa WHERE pa.poll_answer_id = ua.poll_answer_id AND ua.active='Y';

ERROR:  syntax error at or near "DISTINCT" at character 19
LINE 1: SELECT pa.poll_id,DISTINCT(ua.user_id) FROM user_answers ua,...
                          ^
Further if I wrap it in a SUM (with a group by pa.poll_id) I get no error.

I'm a little confused as to what is wrong with my second example there?

--
"Genius might be described as a supreme capacity for getting its possessors
into trouble of all kinds."
-- Samuel Butler