CASE statement and SETOF values

HI!

Consider the following server side function:

CREATE FUNCTION my_function(int4) RETURNS SETOF int8 AS
$BODY$
SELECT
CASE WHEN (some_condition)
THEN (
SELECT ... -- arbitrary select (returning row(s) of int8 values)
)
ELSE (
SELECT ... -- arbitrary select (returning row(s) of int8 values)
)
END
$BODY$
LANGUAGE 'sql' VOLATILE;

This function works fine if one of the two inner SELECT statements returns
exactly one result (one row), but fails whenever one of them returns more
than one result / rows.

What is the reason? I mean the function is declared as returning "SETOF int8",
so why does it expect a scalar?

CU
Christian