to_timestamp error between postgres version 8.3 and 9.2

Dear Team,

I have a table with name registers_info with below columns

regid character varying,
transid bigint,
regdate timestamp without time zone,
canceldate timestamp without time zone

insert into registers_info (regid,transid,regdate,canceldate) values
('reg1',1,to_timestamp('2013-07-24','yyyy-MM-dd
hh24:mi:ss')::TIMESTAMP,to_timestamp('null','yyyy-MM-dd
hh24:mi:ss')::TIMESTAMP);

The above query works fine in Postgres 8.3 version.

But the same query when executed in postgres 9.2 its throwing below error

ERROR: invalid value "null" for "yyyy"
Detail: Value must be an integer.
From application, this cancel date might have values or it may come as null.

Kindly suggest. Thanks in advance.

--
John