"Issue with formula when one field is empty (lookup + total sum problem)"

Hello, I have a simple but very annoying problem.

I have:

  • Field 1: quantity (number)
  • Field 2: price A (a lookup from another table)
  • Field 3: total = quantity * price A (formula)
  • Field 4: price B (another price field)
  • Field 5: total invoice = total + price B

:point_right: The issue:
If price A is empty → total is empty
And because of that, total invoice also stays empty,
even if price B has a value like 5.00€.

I’ve tried everything for hours.
I just want total invoice to show 5.00€ if price B has a value, even if total is empty.

Why is this not working? How can I fix this?
Thanks a lot.