Running total sum doesn't group correctly

Hi! I'm trying to create a report that shows the total registrations by weeks before a conference. Any registrations that happen after the conference date should be listed as "Post-Conference."

The expression in the red box (pictured) is: =IIf(Fields!WeeksOut.Value>0,Fields!WeeksOut.Value,"Post-Conference")
The one in the blue box is: =RunningValue(CountRows(),Sum,"ColumnGroupYEAR")

The problem is the “Post-Conference” registrations won’t group together at the bottom. They display as separate rows. Any ideas on how to fix this?