Crystal Reports Shared Varible Calculation

Message

     Hi,

I am attempting to grand total values from a subreport and a main report.  I have utilized the "Shared Variable" calculation as follows:

WhilePrintingRecords;
Shared NumberVar TTLIND = Sum ({@Duration to number}, {staff_tx_history.STAFFID})

I get a boolean response of False.  I thought the NumberVar set this to a number field.  

What am I doing wrong?

Thanks