pk vs unique not null differences between 8.3.4 and 8.3.8

I prepared a script to upgrade the schema in a 8.3.8 dev box and now
I was trying to replicate the change in a staging 8.3.4 box... and
it failed.

I don't have a recent backup of my dev box so I can't check if I'm
day dreaming.

This statement succede in 8.3.8
alter table shop_commerce_gift drop constraint
shop_commerce_gift_pkey;

but failed in 8.3.4.

I checked the table def in the 8.3.4 and it reports:

giftcode | character varying(16) | not null
...
Indexes:
"shop_commerce_gift_giftcode_key" UNIQUE, btree (giftcode)

but then when I try to drop the index... pg says that the index is
needed for shop_commerce_gift_giftcode_key constraint

was it something that was fixed between the releases or I just
didn't take note of what I really did in the staging box?

btw I was using the same pgsql version from my desktop to \d the
tables.

--
Ivan Sergio Borgonovo
http://www.webthatworks.it