User postgres unable to revoke privileges?

Hi all!

I would like to remove the second line from default privileges, because dataanalysts can't create new tables in public anyway:

# psql -U postgres
psql (9.1.9)
Type "help" for help.

regress=# \ddp
Default access privileges
Owner | Schema | Type | Access privileges
--------------+--------------+-------+-----------------------------------
dataanalysts | dataanalysts | table | dataanalysts=arwdDxt/dataanalysts
dataanalysts | public | table | dataanalysts=r/dataanalysts
svanalytics | public | table | dataanalysts=r/svanalytics
(3 rows)

regress=# ALTER DEFAULT PRIVILEGES FOR ROLE dataanalysts IN SCHEMA public REVOKE SELECT ON TABLES FROM dataanalysts;
ERROR: permission denied for schema public

I'm logged in as postgres, the database superuser. Why am I getting a permission denied?

Thanks!
François Beausoleil