sqlite trigger causes "no such column" exception

I'm a newbie with sql triggers and am getting an ESQLiteException on what seems like a simple example. When I try to modify the "memberTag" column in an existing row, I get the exception "no such column: memberTag". If I drop the trigger, the exception goes away and the row gets updated.

I'm using SQLite and I'm using the "SQLite Expert Personal" app to do this experimenting.

I have this table:

CREATE TABLE [znode] (
  [description] CHAR NOT NULL, 
  [memberTag] CHAR);

and this trigger:

CREATE TRIGGER [memberTagTrigger]
AFTER UPDATE
ON [znode]
FOR EACH ROW
WHEN length(memberTag)=0
BEGIN
update znode
  set memberTag = null;
END;

My update experiment data is something like this:

description  memberTag
one          x
two          (null)

And when I try to change (null) to "y" using SQLite Expert Personal, it throws the exception.