Crystal Reports 10.2 Formula ?

I have a report developed in CR 10.2 and I have a formula field that I am creating. My input dataset is a dynamic dataset with 3 columns in it, 2 of which are defined as decimal. This was defined within a VB.Net 2005 app.

Since adding this formula, I keep getting an error on the formula (I've attached a screen print of the error msg). So, I changed the formula to include a Conversion, however, now when I try and save it, I get another error "A field is required here" on the hightlighted code.

Code:

---

formula = Sum (CDbl ({tblReport.gallons}))

---

Anybody home????