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'