DROP GROUP leaves permissions a mess ...

Using an 8.0.3 database, if I do:

CREATE GROUP testgrp WITH USER pgsql;
GRANT ALL ON timezone TO GROUP testgrp;
DROP GROUP testgrp;

The table permissions still contain the reference to the 'group':
 public | timezone                | table    | {pgsql=arwdRxt/pgsql,"group 100=arwdRxt/pgsql"}

And you can't REVOKE those permissions afterwards:

ams=# REVOKE ALL ON timezone FROM GROUP testgrp;
ERROR:  group "testgrp" does not exist
ams=# REVOKE ALL ON timezone FROM GROUP "100";
ERROR:  group "100" does not exist

Should there not be an ERROR returned when you try and drop a user/group 
that has permissions on a table in the database, to prevent this?