Help using Case/when/then in custom field (dimension) with conditions relating to null values

[bri_c]

We are using looker through Chrome River (Emburse Analytics) and trying to combine 3 fields into one. We have a home department field, an override department field and a sponsoring department field. I way trying to write a condition that would show the override or sponsoring values if present, otherwise default to home department. 

Case(when({sponsoring dept} != null, {sponsoring dept}), {home department})

This is where I started to see if it would populate the correct values before adding all the conditions, it did not. it's definitely on the null part of the statement, because if I change that to look for a specific value, it populates as expected. 

In this statement, how to you write conditions to look for nulls and insert a dimension instead?