elif encoding == 'bpforms.RnaForm':
                            try:
                                subunit_structure = bpforms.RnaForm().from_str(subunit['structure'])
                                sum_length += len(subunit_structure) * bc_form.get_subunit_attribute(subunit_id, 'stoichiometry')
                                bc_form.set_subunit_attribute(subunit_id, 'structure', subunit_structure)