How can I insert NULL into column with the type of timestamp?

H i ,

I want to tansfer a database from sqlserver2005 to postgresql8.2.5.I
use bcp to export a table named admin in sqlserver to a text file:

--table T_admin
id name key regDate
isLock realName
1 rison 998877 2007-08-27 10:24:57 False admin
2 lijun 778899 NULL False NULL
3 guanliyuan 111111 2007-11-05 10:30:08 False myAdmin

--admin.txt
id name key regDate
isLock realname
1 ris 998877 2007-08-27 10:24:57.000 0 admin
2 lij 778899 0
3 guanliyuan 111111 2007-11-05 10:30:08.813 0 myAdmin

I created a table in postgresql,and I use "copy" to import datas

create table T_Admin(
adminID serial not null primary key,
name varchar(30) null,
key varchar(30) null,
regDate timestamp null,
isLock bool null,
realName varchar(30) null
)

copy admin from "/home/postgres/data/admin.txt"

Then error occured:
error:invalid input syntax for type timestamp:""
context:copy T_Admin ,line 2,column regDate:""
It seemed that the column REGDATE cannot accept a NULL.I tested it use:
insert into T_Admin(name,key,regDate,isLock,realName)
values('aaa','aaa','','1','aaa');
The same error occured.

So it means that the column with type timestamp cannot accept a NULL
.Is there any way I can tansfer this table into postgre?How can i deal
with NULL in this case?
Thank you!