Trigger to update other table produces error 1442

[Simon Giddings]

I have three tables in my database which look like this (fields reduced to minimum for readability)

table client
id_client unsigned int
last_update timestamp
...

table client_std
id_client_std unsigned int
id_client unsigned int
...

table client_prem
id_client_prem unsigned int
id_client_std unsigned int

Ok so far.
Now I wanted to update the "last_update" field in the client table when ever it was updated.
So -
create trigger update_client_trigger after update on client
for each row
begin
update client set last_update=now() where id_client=NEW.id_client;
end $$

create trigger insert_poi_trigger after insert on poi
for each row
begin
update client set last_update=now() where id_client=NEW.id_client;
end $$

This worked great, so far so good.

Here is where it gets difficult. I now wanted to update the same table when one of the depending tables (client_std or client_prem) was updated. Here is what I did for the client_prem table :
create trigger updated_client_prem_trigger after update on client_prem
for each row
begin
update client as c join client_std as s on c.id_client=s.id_client
set c.last_update=now()
where s.id_client_std=NEW.id_client_std;
end $$

create trigger inserted_client_prem_trigger after insert on client_prem
for each row
begin
update client as c join client_std as s on c.id_client=s.id_client
set c.last_update=now()
where s.id_client_std=NEW.id_client_std;
end $$

Now, when I do an update on the client_prem table, I get the following message:
"Can't update table 'poi' in stored function/trigger because it is already used by statement which invoked this stored function/trigger."

Can anyone help me resolve this?
Am I going about this in the wrong way ?

Simon