Running Total from a Subform

I have a unbound control box (called textbox 20) in the details section on my
subform that calculates unit price and units orders. The data source looks
like this ...=[UnitPrice]*[UnitsOrdered]

I want to get a running total set up on my main form. Before I do that I'm
pretty sure I need to set up my header on the subform. In my subform, I
entered another unbound textbox (called textbox 26) on the header with a
datasource that looks like this =sum([textbox20]).

Both of these control boxes are set to currency and when I try to preview
the form, my header gives me a #Error.

Thank you for your help!