Tooltip in tablix body (Intersection between Row and Column group)

Hi there

Thanks for sharing your knowledge. One that is driving me mad with Report builder 3.0:
In the same way that I can aggregate data in the intersection between a column group and a row group, I should be able to list those occurrences in the tooltip. But I am not. E.g.

Row group: Product purchased
Column group: Date
Body: Count(Fields!CustomerID.Value)

That gives me the number of customer that purchased a product in an specific day. However if I put JOIN(Fields!CustomerID.Value, ", ") in the Tooltip property, it won't show anything. I also have tried doing a JOIN(lookupset within the same dataset, or a different one, but I get only one customer ID, repeated twice.

Please help!

Thanks