Process Builder: issue when defining criteria for action group

Question

I've created a Process to run a headless flow. I only want the flow to be run when a specific field has changed.

The Account object has a custom lookup field called Associate. The Account Location custom object also has a custom lookup field called Associate. What I'd like to do is update the Associate field in the (child) Account Location objects when it is changed in the (parent) Account.

So, if the Associate field is changed to null, I want the corresponding field in the Account Location objects changed to null. If the Associate field is changed to a value, I want the corresponding field in the Account Location objects changed to that value.

I've tried defining the criteria for the action group several different ways. Each way I do it, the Process returns a error like the following:

Error Occurred During Flow "Update_Associate_on_Account_Location": The flow failed to access the value for...

An error occurred at element myDecision (FlowDecision). The flow failed to access the value for myVariable_current.Associate__c.Id because it hasn't been set or assigned.

My first attempt at the criteria was to specify Conditions:

My second attempt at the criteria as to use a formula:

I then tried to check for null values in the formula, but that produced the same error:

If I select No criteria—just execute the actions!, the Process works fine.

Shouldn't the criteria work regardless of whether the field being checked is set to a value or not?

Answer 1

Don't use the Id value from the related object [Account].Associate__c.Id for validation or to populate the field on child records. [Account].Associate__c lookup field value itself is an Id and you can use it to populate the value (or null)