Trouble with function in approval process

I receive the following error for the formula below in an approval process:

Error:

Function ISCHANGED may not be used in this type of formula

Formula:

ISCHANGED( Addressee_Format__c )

I chose the setting that the process should be used when the formula evaluates to TRUE, and this formula does that. Unsure how to get it to work.

Help is very welcome!

#ApprovalProcess #Formulas #Salesforce Admin