Assign approver from related list

This is something that should be simple but I'm stuck. I am trying to assign an approval to a related user. Object1 has a lookup to Object2. I want to use the Owner of Object2 as the approver when I kick of the approval from Apex. I have tried creating a hierarchical field on the User object called "PLOwner__c", and I assign it to the Owner.ID of Object2 when a submit button is clicked. I get the error:

first error: CIRCULAR_DEPENDENCY, attempt to violate hierarchy constraints: [plowner__c]

Is there an easy way to assign the owner of a related record as next approver from Apex? The Salesforce documentation is confusing.

Bob