tuple error on crosstab with nested columns

[phantomluvs]

Calling all Cognos OLAP / MDX Gurus for help,
I am hit with Cognos RS tuple error. Source is TM1.

I have a crosstab with nested columns and the report is budget vs actual with variances and & differences.  The variance of $$ for months works fine, whereas YTD $$ variance doesn't work for some reason. It only works if I had code the member.
Please refer to the attached image. 

Please feel free if this is not clear or needs more explanation.

Formula Used for Month Calc:
parallelPeriod ([Monthly Income Statement].[t_time].[t_time].[Month],0,[Monthly Income Statement].[t_time].[t_time].[Month]->?Month?)

Formula Used for YTD Calc:
aggregate(currentMeasure within set ( periodsToDate([Monthly Income Statement].[t_time].[t_time].[Year], (parallelPeriod ([Monthly Income Statement].[t_time].[t_time].[Month],0,[Monthly Income Statement].[t_time].[t_time].[Month]->?Month?)))))

Error code:
XQE-PLN-0264
The argument at position 1 of the function 'tuple' is invalid in data item 'YTD Var $' of query 'ReportQuery_ForQrd_ReportQuery.5', expected one of the following types: 'member, measure'.

[phantomluvs]

I figured the solution by changing the aggregate to member