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