CRM_Contact_BAO_Relationship::getRelationship does not return values if relationshipId is passed

Whilst fixing the bug CRM-6895 I ran into the problem that the function CRM_Contact_BAO_Relationship::getRelationship returns an array with Invalid data if the primary key is passed.
If I call it like this: CRM_Contact_BAO_Relationship::getRelationship(0,0,0,0,$relationship_id) it does not return the relation. It does if I do this: CRM_Contact_BAO_Relationship::getRelationship($contact_id,0,0,0,$relationship_id. This does not make any sense as the contact_id does not contribute to idetifying the record?