update syntax error?

Can anybody help with why I am receiving the error below?

Thanks
K.C.

parts=# \d av_parts
Table "av_parts"
Attribute | Type | Modifier
---------------------+-------------+-------------------------------------------------------
itemid | integer | not null default nextval('av_parts_itemid_seq'::text)
vendorid | integer |
partnumber | varchar(25) |
alternatepartnumber | varchar(25) |
nsn | varchar(15) |
description | varchar(50) |
condition | varchar(10) |
quantity | integer |
rawpartnumber | varchar(25) |
rawalternatenumber | varchar(25) |
rawnsnnumber | varchar(15) |
date | integer |
cagecode | varchar(10) |
Indices: av_parts_alternatepartnumber_in,
av_parts_itemid_key,
av_parts_nsn_index,
av_parts_partnumber_index,
av_parts_rawalternatenumber_ind,
av_parts_rawnsnnumber_index,
av_parts_rawpartnumber_index,
av_parts_vendorid_index

parts=# UPDATE av_parts
parts-# SET rawalternaternumber = 'NULL' AND alternatepartnumber = 'NULL'
parts-# WHERE rawalternatenumber = 'IVA81A';
ERROR: left-hand side of AND is type 'unknown', not 'bool'