Sum Function resulting Error in a form

Dear All,

I have a form that set with continuous form. and at the form footer i give a textbox with source a sum formula to sum the data in the detail section. at the first i make the form using the form wizard. after that i place a text box manually to sum the data. at this time there is no error from the sum function. but when i change the textbox name in the detail section the sum formula resulting an #error value. even when change back the textbox name the sum formula still resulting #error.

Please HELP

Regards,
Fauzie