Check if a lookup is null in a formula inside SFM

[jpaccenture]

I have an object that has a lookup field. If the lookup field is null, I want to mark a flag as true. I want to compare it using a formula inside an SFM, but it does not seem to be working. 

I use the following code:

$F.EQUAL($D.Work_Detail__Dependency_Management_.SVMX_NBN_Problem__c , null)

I also changed null to 'null' and didn't work either.

Can someone help me, please?

[ahomoly]

Hi Jessica,

Did you try:

$F.EQUAL($D.Work_Detail__Dependency_Management_.SVMX_NBN_Problem__c , ' ')