Update Trigger in IDS 7.3

Hi, Environment : IDS 7.30UC3 , Sun Solaris This is a question about update triggers. There is a simple trigger: create trigger upd_tab1 update of col2 on tab1 referencing old as pre new as post for each row when (pre.col2 != post.col2) (insert into tab2 (col1,col2) values ('GOT' ,'IT!' )); Then inserting and updating a row in an empty table tab1: insert into tab1 (col1,col2) values('TEST',null); update tab1 set col2 = 'NEW' where col1 = 'TEST'; No rows are inserted in tab2. (Interesting, if either the pre or the post value is NULL, no rows are inserted in tab2) The manual states that if the 'when' evaluates to false/unknown, then the action(insert) will not be done. So, does (pre.col2 != post.col2) evaluate to unknown ??? If yes, then is it expected behaviour ? Confused , and awaiting clarifications... Thanks Madan