Interactive sorting in dynamic columns in a tablix

I am trying to create a report showing revenue per customer per period. I use a tablix in my report. My DataSet is created from a mdx-query. I have defined a row group by Customer called "Customer" and a Column group by periods called "Period". Now I would like to be able to sort by each of the periods presented as columns at runtime. I have not been able to accomplish this yet. In the Text Box Properties of the header of group/column "Period", I have ticked Enable interactive sort on this text box. Sort: group: "Period". Sort by: "Period". This gives me a warning: "The Text Box "Period" is defined at the scope "Period", which is identical to the scope used as SortExpressionScope. This interactive sort has no effect at runtime, since it sorts only the particular instance itself."