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;