update scirpt got ora-01427 when try to update from another table

[sfgaldba]

update application_xml_q2007 a
set a.xml_name = (select b.xml_name from application_xml_info_add b where a.form_id = b.form_id);


still got ORA-01427: single-row subquery returns more than one row

I am not sure how to fix this...anyone? Thanks.