ORA-01858 when INSERT

Hi all,

in 12 C on Win 2008

This INSERT works :

INSERT INTO Ticker (TSTAMP) VALUES ('02/04/11');

But when try to run following I have

INSERT INTO Ticker  VALUES('ACME',TO_DATE('02-Apr-11','dd/mm/yy'),17);

ORA-01858 -  "a non-numeric character was found where a numeric was expected"

Thanks for help.