Cumulative Distinct Count SQL CTools (Pentaho)

Question

I am trying to return a column that calculates the cumulative distinct count of users for each row (i.e. for one row, you are counting the distinct users from that date moving back to the first date/row). When I run this query in DB2, I get "invalid use of keyword DISTINCT".

Any ideas for how to turn this into a working query to return the cumulative distinct count for username?

select title, "Date", th as "Total Monthly Hits", 
sum(th) over (partition by pageid order by "Date") as "Cumulative Total Hits", tuh as "Unique Monthly Hits", 
count(distinct(username)) over (partition by pageid order by "Date") as "Cumulative Total Unique Hits"  from(
select pageid, username, title, ym as "Date", sum(h) as th, sum(uh) as tuh from (
            select pageid, username
            ,NVL((select c.agest from COMMON.VMAX300A_AGENCY_USER c where c.ldap_email=d.username),'Unknown') as agency
            ,title, ym, h, uh from (
                        select pageid, username, title, ym, sum(hits) as h, count(distinct(username)) as uh from (
                                    select PAGEID,a.username
                                    ,(select MAX(b.TITLE) from bfelob_etl.budget_page_hits b where b.PAGEID=a.PAGEID) as title
                                    ,substr(SHORT_DATE,1,7) as ym,
                                    hits
                                    from bfelob_etl.budget_page_hits a
                                    where PAGEID in (1503)
                        ) group by pageid, username, title, ym 
            ) d
) group by pageid, username, title, ym)
where PAGEID in (1503)