Calculate numeric fields

I have a form with two numeric fields (gross_wt, tare_wt). I have a third field fill_wt (set as calculated-read only)

topmostSubform.Page1.Fill_Wt::calculate - (FormCalc, client)

fill_wt=gross_wt-tare_wt

When I open the pdf form, I get

Pls. advise on correct scripting.

Using Acrobat XI Pro, Livecycle ES4