Hello i want to use formula field in ISCHANGED() in Workflow rule criteria but i get error like bellow.

Formula :

IF( AND ( Risk_Rating__c = 'D' , Total_AR__c > 0 ,  ISCHANGED( Oldest_Due_Date__c ) , true, false) )

Note : here Oldest_Due_Date__c is formula field with Date return type.

Error: Function ISCHANGED may not be used in this type of formula