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
OReancode ~ '[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.Elphick@lfix.co.uk
Isle of Wight                              http://www.lfix.co.uk/oliver
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