SQL & PL/SQL :: Conditional Sequence Usage?

I'd like to have a conditional that only gets the next value from a sequence is the current (or supplied) value is null.

Here are some trivial examples:

CREATE SEQUENCE ts
MINVALUE 1
START WITH 1
INCREMENT BY 1;
SELECT ts.NEXTVAL FROM DUAL;
SELECT COALESCE(ts.CURRVAL, ts.NEXTVAL) FROM DUAL;
SELECT NVL(ts.CURRVAL, ts.NEXTVAL) FROM DUAL;
SELECT CASE
WHEN ts.CURRVAL IS NOT NULL THEN ts.CURRVAL
ELSE ts.NEXTVAL
END
FROM DUAL;

Ideally multiple executions of any of these (or a better one of your design) should return the same value from the sequence, but mine do not.