How to use substring or something similiar in dimensional data

[mmestan]

Hi, I'm using Cognos 11 and dimensional report with tree prompt.
I'm using data of it in a Combination chart as Y axis, but I need to short the whole description of each value in each level to shorten version. I'm using this expression substring([MAIN_graf_alt].[PC - set],position('-',[MAIN_graf_alt].[PC - set]) +1, -1) . It is working in many charts, but as soon as I put this chart into repeater and trying deep into lover level of hierarchy by using values in charts,  I'm getting error :
CRX-API-0005 An error occurred at or near position '10' The variable named '[MAIN_graf_alt].[childre((here is name of children))]' is invalid. CAF-WRN-2082 ....