Result of query has a [Function] as value

Question

I'm running a SQL statement and it has a sub select. But after running, the result field has a [Function] value.

Do I doing something wrong? My SQL is something like it:

SELECT
    product.CODE
    , (SELECT LIST(item.size, '/') FROM ITEMS item WHERE item.CODE = product.CODE) AS SIZES
FROM
    PRODUCTS product
WHERE
    product.CODE = '98015';

But the result is:

CODE	SIZES
98015	[Function]

Can someone help me?