RDLC sub report ----Object reference not set to an instance

Hi,
We have rdlc reports in our application.Have some reports with sub reports.
Problem is we are getting 
object reference not set to an instance of an object. error in rdlc main report
we are adding a Parent group on Detail (which have sub report in it)
when visibility toggle is added to this group we are getting the above error.


please help me .......its urgent....