Sum If function in textbox

[arothacker]

I have a form called "Headcount" with the fields "EmployeeType" and "Count"

How do I write an equation in a textbox at the type of the form that sums the "Count" field when the "EmplyeeType" = Salary?

I've tried putting =Sum(IIf([EmployeeType]="Salary",[Count],0)) but I get #Error

Thanks!