Simple Infomaker question, I think

I am trying to modify the Fund Activity report.

What I have done so far:

1.  Copied standard report to ld_fund_activity in the local.pbl file.

2.  Removed the excess fields

3.  Created a header for the new column, Pledge + Gift - Write Off

4.  Added a text box for the calculated data

5.  in the Expression builder I have this Expression, that when I verify it says it is OK:

     (  pledge  +  gift  -  write_off  )

The report shows nothing in this column.  I am so confused.