Error in updating an updatable snapshot

I am testing the updatable snapshot feature in Oracle 7.2. I have created a master site and a snapshot site. The replication from master site to snpashot site works well. But when I tried to update the updatable snapshot on the snapshot site. It gave me errors:

 SQL> update emp set sal=0.0;
update emp set sal=0.0

                   *

ORA-04068: existing state of packages has been discarded
ORA-04063: package body "SYS.DBMS_DEFER" has errors
ORA-06508: PL/SQL: could not find program unit being called
ORA-06512: at line 13
ORA-04088: error during execution of trigger 'IOUGDEMO.EMP$RT'

I basically fowllowed the demo in "Oracle Server Distributed System, volumn II: Replicated Data." to perform this test.

Any information on this will be greatly appreciated.

-- 
Bo Lu
Senior Programmer/Analyst
Hospital and Clinical Informatics, U. of Utah
801-581-5964, bo.lu_at_hsc.utah.edu