how to add primary key in exsiting table in sql server 2005(query)

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SQL-Server-2005

SQL-Server

Hello

i have execute the following query on sql editor

create database Employee
use Employee
create table Emp
(
id integer,
E_Name nvarchar (20),
e_Add nvarchar(50)
)
select * from Emp
insert into Emp values('001','Reshma','BBSr')

so i want to add a primary key on existing table Emp
i have write the following query on sql editor

alter table Emp add constraint pk_Emp primary key(id)

but after execute the of the above query

the following error will be raised

Msg 8111, Level 16, State 1, Line 1
Cannot define PRIMARY KEY constraint on nullable column in table 'Emp'.
Msg 1750, Level 16, State 0, Line 1
Could not create constraint. See previous errors.

So how it can possible please suggest me ASAP