Why not possible to Sum calculated field in the footer of a subform?

Example:

There is a product order 10000 pcs, which is delivered couple times.

One pallet contains 100 produts.

One occasion the lorry takes for example 12 pallets.

Form ORDER contains subform DELIVERY.

PALLETPCS is a field in DELIVERY table.

In the footer of subform Sum([PALLETPCS]) will work.

If I have a calculated field like PRODUCTPCS = PALLETPCS * 100

then Sum([PRODUCTPCS]) will show #Error

In fact Sum([PALLETPCS]) will also show #Error

I do not understand this. Why??

Question:

How can I summarize a calculated data in the footer of a subform??

Please help. Thank you