Cube Designer, In TOP_N Expression, the choice of SUM|ORDER BY Column and Group By Column are all measures column

[????]

Hi
 When i designer cube, in topn measures, i should choose a measure column in 
SUM|ORDER BY Column and choose a dimension column in Group BY Column, But I 
only can choose a measure column in all SUM|ORDER BY Column and  Group By 
Column.


Is it a bug in shown, how can solve it? Thanks!
  
ORDER | SUM BY Column
     




Group By Column: