Parent Record cannot be returned to from new Child Record

In a parent record I have a formula url field (button) that creates a child record.  I am using the api_GenAddRecord because the user needs to enter in information and then when they click save they are supposed to return to the parent: URLRoot() & "db/" & [_DBID_CHILDTABLE] & "?a=API_GenAddRecordForm&_fid_6=" & URLEncode ([key_field_not_recordID) & "&z=" & Rurl().  It takes me to the child record but after I click save it says "Parent Record not found It has probably been deleted. If you followed a link to get here, you may want to inform the author of that link that this material no longer exists."

I have gotten this to work in another table but in that case I was using the recordID not a key field and I am wondering if that is the issue.