Relationship_type_id does not use pseudoconstants

An API call "relationship, create" with the $params array containing a relationship_type_id parameter of type string throws an error saying it must be of type Integer.

As far as I understand, behavior should be that a relationship_type_id of type String should be converted to an Integer using the pseudoconstants, so for example relationship_type_id = 'Employee of' should be automatically converted to relationship_type_id=5 by the API before processing the call.