SQL Report Builder 3.0 LookupSet Function on Sharepoint Lists

I have two sharepoint list named city and town. I have created two datasets for each list in SQL Repor Builder 3.0 . City dataset has ID and Title columns. Town dataset has ID, City_Id and Title.

My purpose is showing cityname and it's towns in same row. I use LookupSet function like this; =Join(LookupSet(Fields!ID.Value, Fields!City_ID.Value, Fields!Title.Value,"Town"), ",")

but, it gives just blank field. How I can use LookupSet function. Thanks