Can I use subselect as a function parameter?

I want to do this:

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

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, product
WHERE brandname.id = product.brand
), eancode)
END
)
);

The parser accepts it, but when it is run on a line that matches the ELSE in the constraint, I get:

copy product from '/usr1/avoca/dumps/dbdump.product'
ERROR: copy: line 2, ExecEvalExpr: unknown expression type 108

Can this be made to work at all (in 7.0.2)?

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