I receive the following error when I try to merge 2 contact records: INVALID_FIELD_FOR_INSERT_UPDATE, Contact: bad field names on insert/update call: Comps__r: 

The records the I am trying to merge are generated from the following 2 queries:

This will give me the childs in the merge...the record that will get deleted:

Contact childCon : [select Id, Acc_No_Part_1__c, Acc_No_Part_2__c, MergeMasterID__c, (select Id, Contact__r.MergeMasterId__c, External_ID__c from Comps__r) from Contact where Id IN :childCons)]

 This gets me the parents:

(Contact masterCon : [select Id, Acc_No_Part_1__c, Acc_No_Part_2__c, MergeMasterID__c, (select Id, Contact__c, Contact__r.MergeMasterId__c, External_ID__c from Comps__r) from Contact where Id IN :masterConSet])

 Before I can merge them I have to manipulate the child records found Camps__r. Once those updates are done I run a merge statement.

I think the error happens only when the Child Contact does not have any values in Comps__r. My code should be handling that scenario...In other words I use if statements to test if there are records in Comps__r, and if there is isn't then I have code to handle it differently.  

When the child contact has records in comps__r then everything works ok. 

The other posts that I have found related to this message are not helping, and the definition for the message in docs seems to be way off the mark:

INVALID_FIELD_FOR_INSERT_UPDATE

You can't combine a person account record type change with any other field update.

So, I am stumped at this point.