Add Total Line To Report

I have a report in my trucking application that reports information at the state (NJ, NY, VT etc..)level.                    
                             Total Miles  Fuel Purchased  MPG    Fuel Used
For example:  NJ      300                  10.0             5.0      60.0 (which is a calculation, 300/5)
                       NY     400                  50.0             5.0     80.0
           etc...
Totals                       700                   60.0                       140.0

The user now wants to break the information down further to know the State information by Driver.  I revised the report to add a driver grouping prior to the state grouping.  I am now creating a new line for the driver group footer.  I want to add a total of all 'Fuel Used' for each driver.  Fuel Used is a calculated field called 'FuelUsed' on the state footer.  The source of the field is '=[TotMi]/[MPG]'.  "TotMi' and MPG are two other fields on the state footer.

In the driver footer I have an unbound field called 'DriverFuelUsed' which I want to be the total of all of the states this driver purchased fuel in.  I make the source of my 'DriverFuelUsed' field =Sum([FuelUsed]).  I even cut and paste the string 'FuelUsed' from the state footer to make sure I have it correct.

Every time I run the report is prompts me to enter 'FuelUsed', like that field doesn't exist or at least it can't find it.  What am I doing wrong?