Count distinct on any dimension should work even not a predefined measure

Currently count distinct on a dimension does not work:

{"sql":"select DATE'2015-07-18' , count(distinct country) as uniquecountry from pc_session e INNER JOIN pc_cal c ON e.part_date = c.cal_dt WHERE (part_date BETWEEN DATE'2015-07-18' AND DATE'2015-08-19') ","offset":0,"limit":10,"acceptPartial":true,"project":"tracking"}

"exception": "Can't find any realization. Please confirm with providers. SQL digest: fact table DEFAULT.PC_SESSION,group by [],filter on [DEFAULT.PC_SESSION.PART_DATE],with aggregates[FunctionDesc [expression=COUNT_DISTINCT, parameter=ParameterDesc [type=column, value=COUNTRY], returnType=null]].
while executing SQL: "select DATE'2015-07-18' , count(distinct country) as uniquecountry from pc_session e INNER JOIN pc_cal c ON e.part_date = c.cal_dt WHERE (part_date BETWEEN DATE'2015-07-18' AND DATE'2015-08-19') LIMIT 10""