Can't Sum formula field?

I have a formula field that I display with my group totals in my report, but for some reason I can't show a sum for it at the report footer with the rest of my grand totals.

This is the contents of my formula field that I need to summarize:

Code:

---

if trim({Agent.Agent_Tax_Id}) <>  "" then
    {Can_Tax_Percentage.Can_Tax_Percentage_GST} * {#GSTAble} - Sum ({Agent_Fee.Agent_Fee_Tax})
else
    0

---

edit: this is in crystal Xi

Unless things have changed in version XI, you cannot Sum a formula that contains a Sum.

Sum ({Agent_Fee.Agent_Fee_Tax}) is already a Grand Total.