Function with Array

Hello to all... I am with a problem in a function that I am creating. The
parameters that are function go to receive are an ARRAY and the return also
will be an ARRAY, the entrance parameters will be used in a clause SQL. E
the exit ARRAY will be the result of the SQL. However, I elaborated the
function and independent of the value that played in the ARRAY the result of
the SQL was always null. To make a test I elaborated the function below and
I perceived that my ARRAY had the null values.

CREATE OR REPLACE FUNCTION TB_ADM_CEP_SEL (numeric [])
RETURNS numeric [] AS
$body$
DECLARE
BEGIN
--SELECT "TX_CEP" FROM "SMART"."TB_ADM_CEP"
--WHERE "PK_IN_COD_CEP" = $1[0];
RETURN $1[0];
END;
$body$
LANGUAGE 'plpgsql' VOLATILE RETURNS NULL ON NULL INPUT SECURITY INVOKER;
Somebody knows what I am making of made a mistake in this function to have
these results? They forgive my English good and not very obliged for the
suggestions.
Thank you...