Postgres - Return function in Case When Statement

Hey i want to create a random number. When this random number already exist i want to call the randomnumber function again and again. In that case i need to return the randomnumber function inside in CASE WHEN statement. It does not work. Still get error that number already exist. I want to create a random number for unique column:

CREATE OR REPLACE FUNCTION getrandomnumber(integer, integer)
        RETURNS integer AS
        $BODY$
        DECLARE
            start_int ALIAS FOR $1;
            end_int ALIAS FOR $2;
            name int;
        BEGIN
        name = trunc(random() * (end_int-start_int) + start_int);

        CASE WHEN (select count(alias) from drivers where alias = name limit 1) = 0

        THEN RETURN name;

        ELSE RETURN getrandomnumber(start_int, end_int);

        END CASE;
        END;

        $BODY$

        LANGUAGE plpgsql VOLATILE STRICT
        COST 100;
        ALTER FUNCTION getrandomnumber(integer, integer)
        OWNER TO postgres;