"Invalid Operation" in Report

I have a query that is pretty straightforward and I am trying to
generate 5 reports from that query. The reports produce totals by
Project Manager, Category, Client, Account Manager and Account
Executive. The first three reports work fine. However, when I try to
change the sorting and grouping to either Account Manger or Account
Executive, I get an "Invalid Operation" error and the report does not
open.

Has anybody seen this before?

Thanks in advance.