sqlite trigger causes "no such column" exception -

i'm newbie sql triggers , getting esqliteexception on seems simple example. when try modify "membertag" column in existing row, exception "no such column: membertag". if drop trigger, exception goes away , row gets updated.

i'm using sqlite , i'm using "sqlite expert personal" app experimenting.

i have table:

create table [znode] (   [description] char not null,    [membertag] char); 

and trigger:

create trigger [membertagtrigger] after update on [znode] each row when length(membertag)=0 begin update znode   set membertag = null; end; 

my update experiment data this:

description  membertag 1          x 2          (null) 

and when try change (null) "y" using sqlite expert personal, throws exception.

the problem in when clause: database not know membertag comes from, because there two possible rows, old one, , new one.

use either old.membertag or new.membertag.

(there problem: update change all rows in table, because forgot clause.)