Using Case statements in List in Cognos Analytics

[fakxy]

Hello All,

I am preparing a list report. I am trying to use a data item and need to group the list on that. I am using a case statement for that data item. But when I am trying to group on that, it is throwing an error.

 "XQE-PLN-0040 The query is not supported. Tuple-based value expressions are not supported as grouping items in list reports. Data item name = 'CEU EEU SEU Group' , valueset name = '7'.

Cognos Version - 11.1.7
Model - DMR

Any quick help is highly appreciated.

Regards,
Fakeha