Is this a bug in the table definition or in PostgreSQL?

The complicated constraint in this definition of the product table doesn't
work, but should it?  or is there a better way to do this?

CREATE FUNCTION ean_checkdigit(bpchar, bpchar) RETURNS BOOLEAN     AS '/usr1/proj/bray/sql/funcs.so' LANGUAGE 'C';

CREATE TABLE brandname
(  id           CHAR(12)        PRIMARY KEY,  ean_prefix   CHAR(8)         CHECK (ean_prefix ~ '50-[0-9]{5}')
                   REFERENCES company(ean_prefix)                                       ON UPDATE CASCADE
                       ON DELETE NO ACTION,  name         TEXT            NOT NULL
 
)
;

CREATE TABLE product
(  id                   CHAR(10)        PRIMARY KEY,  brand                CHAR(12)        REFERENCES brandname(id)
                                         ON UPDATE CASCADE                                               ON DELETE NO
ACTION, eancode              CHAR(6)         CHECK (eancode IS NULL                                           OR
eancode~ '[0-9]{6}'),  ...  CONSTRAINT ean CHECK (        CASE WHEN eancode IS NULL OR brand IS NULL
THEN't'                  ELSE ean_checkdigit(                      (SELECT ean_prefix                         FROM
brandname                        WHERE brandname.id = brand                      ), eancode)        END     )
 
)
;

copy product from '/rover/avoca/dumps/dbdump.product'
ERROR:  copy: line 2, ExecEvalExpr: unknown expression type 108
[line 1 had null values in the relevant fields]

-- 
Oliver Elphick                                Oliver.Elphick@lfix.co.uk
Isle of Wight                              http://www.lfix.co.uk/oliver              PGP key from public servers; key
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