#ERROR in webi cell, though it's parsing OK

I have a variable which parse OK but its giving #errror in Webi report

=If(Pos(ReportFilterSummary();"Rep Period Equal Current Month")>0;[Rep Period];ToNumber(Substr(Substr(ReportFilterSummary();Pos(ReportFilterSummary();"Rep Period Equal");25);18;7)))

Please suggest any solution