Two timestamp minus and cannot cast to interval value

SQL>select cast((timestamp'2017-04-06 11:50:00' - timestamp'2017-04-06 11:45:00') as interval second) from (values(1));

(EXPR)
----------
0.000000

— 1 row(s) selected.

SQL>select cast((timestamp'2017-04-06 11:50:00' - timestamp'2017-04-06 11:45:00') as interval minute) from (values(1));

(EXPR)
------
0

>>select timestamp'2017-04-06 11:50:00' - timestamp'2017-04-03 11:45:00' from dual;

(EXPR)
-------------

3

— 1 row(s) selected.