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.