can insert 'null' into timestamp type field from command line but not from input file?

The table is like this:

========
maxware=# \d test;
Table "public.test"
Column | Type | Modifiers
--------+-----------------------------+-----------
a | timestamp without time zone |
b | integer |
=========

The following insert command works fine:

maxware=# insert into test (a,b) values (null,'1');

But importing from an input data file does not seem to work as shown below:

=========
maxware=# copy tbl_spcase from '/home/bdu/test/input.data' delimiter as '|';
ERROR: invalid input syntax for integer: "null"
CONTEXT: COPY tbl_spcase, line 1, column col_id_spcase: "null"
==========

The input.data file just has one line. But no matter what I did, neither
of the following input format worked.

1. null|1
2. |1
3. ''|1

How should I represent blank value for the field that's of timestamp type?

Thanks in advance for any help,

Bing