Is this a bug? (changing sequences in default value)

Pg 8.1.11, I try to change sequences as default value of a table, then
remove old sequence:

# \d table1
Table "table1"
Column | Type | Modifiers
--------+---------+---------------------------------------------------------------
id | integer | not null default nextval('table1_id_seq'::regclass)
nombre | text | not null
Indexes:
"table1_pkey" PRIMARY KEY, btree (id)

# ALTER TABLE table1 alter column id set default nextval('newseq_id_seq');
ALTER TABLE

# \d table1
Table "table1"
Column | Type | Modifiers
--------+---------+---------------------------------------------------------------
id | integer | not null default nextval('newseq_id_seq'::regclass)
nombre | text | not null
Indexes:
"table1_pkey" PRIMARY KEY, btree (id)

# drop SEQUENCE table1_id_seq ;
ERROR: cannot drop sequence table1_id_seq because table
table1 column id requires it
HINT: You may drop table table1 column id instead.

Am I doing something wrong?

Thanks!

Fernando.