Difference Pivot Table Empty Cell

Hello,

I am trying to calculate the difference between units sold of some products on 2 consecutives weeks, the problem is that, at some weeks, my table has not any register of a sold, so, on the pivot table some lines are empty. Because of this, when I try to do the difference between then, it returns me a null value.

Example

Hello,
You need to convert nulls to 0s before.
changenull = ifelse(isNull(Units),0,Units)