Hello, I am trying to build a process builder that will update a custom field "Approved By" when the "Approved?" field is set to TRUE.    I have done something similar on the Account object and it has worked but when I do this on the Work object from Agile I am getting an error.

My immediate action is Field "Approved By" Type "Formula" and Value "[agf__ADM_Work__c].agf__Last_Modified_By__c.FirstName & " " & [agf__ADM_Work__c].agf__Last_Modified_By__c.LastName 

When I try to save a work item, I receive this error: This error occurred when the flow tried to update records: The flow failed to access the value for myVariable_current.agf__Last_Modified_By__c.FirstName because it hasn't been set or assigned..

On the Account object, I have a similar process builder with: [Account]. at the beginning rather than [agf__ADM_Work__c].

Is there any reason why the process works on Account but I am struggling on the Work object?