Conditional Formatting of Blank Cells not working

Summary

I want Null / Blank Cells in Red

Content

Hi,

I have a report (see XML attached) that has 17 columns, some of which have missing data (NULLS).

All I am trying to do is conditionally format any cell that is NULL to be RED - it doesn't work for Job Name, Manager Name, Dept & some other fields.

I've managed to successfully change blank cells for the Grade column but here the condition I used  is = IS NOT IN  (we only have a set amount of grades so that approach can work here. This won't work for Manager Name, Job Name or Dept as these are changing too frequently.