Transaction Error

I dont want to insert nulls in my PK column but i thought the ID would generate automatically by using the @@Idenity field...can someone help me out on where I am going wrong?

{"Cannot insert the value NULL into column 'transactionId', table 'LibraryManager.dbo.BookOrder'; column does not allow nulls. INSERT fails.
The statement has been terminated."}
[System.Data.SqlClient.SqlException]: {"Cannot insert the value NULL into column 'transactionId', table 'LibraryManager.dbo.BookOrder'; column does not allow nulls. INSERT fails.
The statement has been terminated."}




ALTER PROCEDURE [dbo].[SaveBookOrder]

@transactionId int,

@userId int,

@bookId int,

@checkedoutDate DateTime,

@checkedinDate DateTime,

@Result int output



as

-- proc settings

SET NOCOUNT ON

-- begin trans

BEGIN TRANSACTION



begin


update BookManagement

set isCheckedIn = 0

where bookid = @bookid


-- check for error

IF @@ERROR <> 0

GOTO ErrorHandler

else

set @transactionId = @@Identity

insert BookOrder(userID, bookId, checkedOut, checkindate)

values(@userID, @bookId, @checkedoutDate, @checkedinDate)

end

-- commit transaction, and exit


set @Result = @transactionId

COMMIT TRANSACTION

RETURN 0



-- Error Handler

ErrorHandler:

-- see if transaction is open

IF @@TRANCOUNT > 0

BEGIN

-- rollback tran

ROLLBACK TRANSACTION

END

-- set failure values

SET @Result = -1

RETURN -1