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(dot)Elphick(at)lfix(dot)co(dot)uk
Isle of Wight http://www.lfix.co.uk/oliver
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