In access form sum of calculated field

[pgdeval]

I have designed a form for purchase register. In that the quantity * rate is shown in amount column by formula. I want to sum the amount column but when auto sum is used it does not show sum function. I also tried to enter formula i.e. =Sum [Amount] but it does not work. Can any body suggest solution to this?

[June7]

Use parens.

=Sum([Amount])

Textbox needs to be in form header or footer section.