database logon trigger woes

Anyone seen this anomaly...I was aiming to automatically enable tracing on the 'demo' schema as they logged on...

create or replace
trigger demo_trace
after logon on demo.schema
begin
  dbms_session.set_sql_trace(true);
end;
/

compiles no problems... but then

SQL> conn demo/demo
ERROR:

ORA-00604: error occurred at recursive SQL level 1
ORA-01031: insufficient privileges
ORA-06512: at "SYS.DBMS_SESSION", line 83
ORA-06512: at line 4

Warning: You are no longer connected to ORACLE.

SQL> conn sys
Enter password:
Connected.
SQL> drop trigger demo_trace;

Trigger dropped.

SQL> conn demo/demo
Connected.
SQL> set role none;

Role set.

SQL> exec dbms_session.set_sql_trace(true);

PL/SQL procedure completed successfully.

Even more interestingly was if I changed it to 'alter session' using dbms_sql then it still didn't work, but OTHER alter session commands (such as current_schema) worked fine...

Hmmmmmmmmmm..........

-- 
===========================================
Connor McDonald
http://www.oracledba.co.uk

We are born naked, wet and hungry...then things get worse