Why is Tableau returning Null for this Calculated Field

Question

I have a calculated field trying to calculate a simple percentage between two fields that is returning NULL

I have a calculated field trying to calculate a simple percentage between two fields:

Sum([Field1]) returns values for all rows Sum([Field2]) returns values for all rows Sum([Field1])/Sum([Field2]) returns NULL for all rows`

This makes no sense to me. And even more infuriatingly Sum([Field1])*(1/Sum([Field2])) returns the percentage as expected

So I'm at a loss as to why the simple percentage value is returning a NULL