Whats wrong with this logon trigger?

Hi,
Whats wrong with this, I get error regarding row 5 (alter session)

  CREATE OR REPLACE TRIGGER SYS.set_trace 
  AFTER LOGON ON DATABASE 
  WHEN (USER like  'PETE') 
  DECLARE     
  lcommand varchar(200); 
  BEGIN     
  EXECUTE IMMEDIATE 'alter session set statistics_level=ALL';      
  EXECUTE IMMEDIATE 'alter session set max_dump_file_size=1000M';     
  EXECUTE IMMEDIATE 'alter session set events ''10046 trace name context forever, level 12'''; 
  END set_trace; 
  /

Well, trying to do this throws an error:

alter session set max_dump_file_size=50M;

ORA-02248: invalid option for ALTER SESSION


Regards
Peter