Unique Key Problem

Question

Database : SQL SERVER 2005
Front-End : Visual Studio 2005
I create table T1 with columns : EMPNUM numeric(3,0),EMPName varchar(25)

EMPNUM is unique key.

I insert NULL value in EMPNUM.

It allows inserting of NULL value once only then it will give error about violating of Unique Key.

If you have solution let me know in brief as soon as possible.